I found this amazing summary from GMAT Genius's review of the book on amazon:

OVERVIEW of OFFICIAL GUIDE 2018

The Official Guides for GMAT Review contain retired real GMAT questions, and are an essential component of your GMAT preparations. The GMAC places questions in order of increasing difficulty, based on its assessment of difficulty. The Quantitative Official Guide has no overlap with questions in the main Official Guide.

The 2018 edition of this book contains 45 new questions out of the 300 total questions, representing 15% new content. These are new questions that we have not encountered before; they are not questions recycled from older GMAC resources.

PROBLEM SOLVING

This guide contains 176 Problem Solving questions. The GMAC classifies question difficulty into three categories as follows:

Easy - 89 (51%, 4 more than 2017)

Medium - 43 (24%, 9 fewer)

Hard - 44 (25%, 5 more)

The Problem Solving section contains 26 new questions, with difficulty of Easy / Medium / Hard as follows: 15 / 5 / 6. This is in lieu of 26 questions from the 2017 edition that have been removed, with difficulty of Easy / Medium / Hard as follows: 11 / 14 / 1. Unlike in prior years, the GMAC has not reclassified the difficulty of any question.

GMAT Genius classifies question difficulty into five categories. Our assessment skews slightly more towards the center, but contains notable differences from the GMAC. Our difficulty assessment is only 76.1% correlated with the GMAC’s assessment, clearly showing that there is subjectivity involved in assessing question difficulty. Here’s our breakdown:

Super Easy - 21 (12%, 1 fewer than 2017)

Easy - 55 (31%, same)

Medium - 62 (35%, 1 fewer)

Hard - 30 (17%, 1 more)

Very Hard - 8 (5%, 1 more)

Although many math questions entail multiple math concepts, GMAT Genius classifies questions based on our assessment of the primary math concept. We break down the 176 Problem Solving questions as follows:

Arithmetic - Basic: 16 (9.1%, 2 more than 2017)

Arithmetic - Absolute Value: 3 (1.7%, same)

Arithmetic - Divisibility/Factors/Mult.: 7 (4%, 3 fewer)

Arithmetic - Exponents & Roots: 8 (4.5%, 1 fewer)

Arithmetic - Fractions & Ratios: 22 (12.5%, 1 more)

Arithmetic - Percents: 12 (6.8%, 3 fewer)

Arithmetic - Pos/Neg & Odd/Even: 1 (0.6%, 2 fewer)

Algebra - Inequalities: 4 (2.3%, 1 more)

Algebra - Linear Equations: 9 (5.1%, same)

Algebra - Quadratics: 10 (5.7%, 2 more)

Algebra - Simultaneous Equations: 6 (3.4%, same)

Algebra - Variables in Answers: 5 (2.8%, 1 more)

Geometry - Circles: 4 (2.3%, same)

Geometry - Coordinate: 5 (2.8%, same)

Geometry - Rectangles: 5 (2.8%, same)

Geometry - Triangles: 4 (2.3%, same)

Geometry - Other: 4 (2.3%, same)

Statistics - Averages: 14 (8%, 1 more)

Statistics - Other: 3 (1.7%, same)

Word Problems - Combinatorics: 4 (2.3%, same)

Word Problems - Functions & Sequences: 12 (6.8%, same)

Word Problems - Groups/Sets: 3 (1.7%, 1 more)

Word Problems - Probability: 2 (1.1%, same)

Word Problems - Revenue/Profit/Interest: 5 (2.8%, 1 more)

Word Problems - Rate & Work: 8 (4.5%, 1 fewer)

Here’s a list of the 26 new Problem Solving questions: 13, 22, 29, 31, 52, 55, 65, 70, 71, 73, 74, 75, 78, 85, 89, 92, 99, 102, 104, 117, 135, 141, 144, 156, 165, 168

Here’s a list of the 176 Problem Solving questions categorized by primary math concept:

Arithmetic - Basic: 2, 5, 21, 44, 45, 67, 75, 77, 85, 88, 89, 92, 94, 100, 162, 170

Arithmetic - Absolute Value: 95, 105, 150

Arithmetic - Divisibility/Factors/Multiples: 52, 86, 103, 114, 122, 171, 173

Arithmetic - Exponents & Roots: 34, 36, 38, 48, 90, 112, 143, 160

Arithmetic - Fractions & Ratios: 8, 20, 24, 35, 40, 43, 46, 49, 54, 55, 60, 64, 73, 74, 78, 83, 84, 91, 108, 136, 144, 176

Arithmetic - Percents: 7, 17, 28, 30, 47, 59, 99, 111, 117, 118, 132, 168

Arithmetic - Pos/Neg & Odd/Even: 14

Algebra - Inequalities: 4, 29, 101, 163

Algebra - Linear Equations: 3, 9, 13, 15, 18, 42, 50, 71, 98

Algebra - Quadratics: 1, 19, 22, 33, 63, 66, 110, 121, 128, 156

Algebra - Simultaneous Equations: 12, 25, 31, 32, 41, 133

Algebra - Variables in Answers: 16, 39, 102, 120, 125

Geometry - Circles: 37, 134, 151, 169

Geometry - Coordinate: 26, 87, 96, 109, 115

Geometry - Rectangles: 11, 27, 104, 129, 175

Geometry - Triangles: 62, 68, 140, 164

Geometry - Other: 23, 139, 146, 149

Statistics - Averages: 10, 58, 61, 82, 93, 107, 119, 126, 131, 135, 147, 148, 153, 161

Statistics - Other: 72, 79, 81

Word Problems - Combinatorics: 152, 154, 155, 157

Word Problems - Functions & Sequences: 53, 56, 69, 76, 80, 116, 127, 137, 138, 145, 159, 166

Word Problems - Groups/Sets: 106, 141, 142

Word Problems - Probability: 158, 167

Word Problems - Revenue/Profit/Interest: 6, 65, 124, 165, 172

Word Problems - Rate & Work: 51, 57, 70, 97, 113, 123, 130, 174

DATA SUFFICIENCY

This guide contains 124 Data Sufficiency questions. The GMAC classifies question difficulty into three categories as follows:

Easy - 27 (22%, 1 more than 2017)

Medium - 33 (27%, 2 more)

Hard - 64 (52%, 3 fewer)

In the Data Sufficiency section, questions are not fully presented in order of progressive difficulty, contrary to what the back cover of the book claims. Based on difficulty levels provided in the online version, Medium and Hard difficulty questions are interspersed. The following lists the question numbers for each difficulty level:

Easy - 177-203

Medium - 204-235, 239

Hard - 236-238, 240-300

The Data Sufficiency section contains 19 new questions, with difficulty of Easy / Medium / Hard as follows: 2 / 6 / 11. This is in lieu of 19 questions from the 2017 edition that have been removed, with difficulty of Easy / Medium / Hard as follows: 1 / 4 / 14. The GMAC has not reclassified the difficulty of any question.

GMAT Genius classifies question difficulty into five categories. Our assessment skews significantly towards the easier side, and contains notable differences from the GMAC. Our difficulty assessment is only 68.5% correlated with the GMAC’s assessment, demonstrating tremendous subjectivity involved in assessing question difficulty. Here’s our breakdown:

Super Easy - 12 (10%, 1 more than 2017)

Easy - 25 (20%, 3 fewer)

Medium - 54 (44%, same)

Hard - 27 (22%, 2 more)

Very Hard - 6 (5%, same)

Although many math questions entail multiple math concepts, GMAT Genius classifies questions based on our assessment of the primary math concept. We break down the 124 Data Sufficiency questions as follows:

Arithmetic - Basic: 3 (2.4%, 2 fewer than 2017)

Arithmetic - Absolute Value: 1 (0.8%, same)

Arithmetic - Divisibility/Factors/Mult.: 8 (6.5%, 2 more)

Arithmetic - Exponents & Roots: 12 (9.7%, 1 fewer)

Arithmetic - Fractions & Ratios: 8 (6.5%, same)

Arithmetic - Percents: 6 (4.8%, same)

Arithmetic - Pos/Neg & Odd/Even: 5 (4%, same)

Arithmetic - Primes: 3 (2.4%, 1 more)

Algebra - Inequalities: 12 (9.7%, 2 more)

Algebra - Linear Equations: 3 (2.4%, same)

Algebra - Quadratics: 7 (5.6%, 2 more)

Algebra - Simultaneous Equations: 9 (7.3%, 5 fewer)

Geometry - Circles: 6 (4.8%, 1 more)

Geometry - Coordinate: 4 (3.2%, same)

Geometry - Rectangles: 3 (2.4%, same)

Geometry - Triangles: 3 (2.4%, same)

Geometry - Other: 2 (1.6%, same)

Statistics - Averages: 6 (4.8%, same)

Statistics - Other: 5 (4%, same)

Word Problems - Functions & Sequences: 7 (5.6%, same)

Word Problems - Groups/Sets: 4 (3.2%, 1 more)

Word Problems - Probability: 1 (0.8%, 1 fewer)

Word Problems - Revenue/Profit/Interest: 2 (1.6%, same)

Word Problems - Rate & Work: 4 (3.2%, same)

Here’s a list of the 19 new Data Sufficiency questions: 178, 193, 215, 216, 224, 226, 231, 239, 244, 246, 247, 257, 270, 281, 283, 285, 291, 295, 296

Here’s a list of the 124 Data Sufficiency questions categorized by primary math concept:

Arithmetic - Basic: 177, 180, 234

Arithmetic - Absolute Value: 214

Arithmetic - Divisibility/Factors/Multiples: 221, 224, 238, 247, 257, 272, 296, 297

Arithmetic - Exponents & Roots: 184, 210, 226, 240, 251, 254, 267, 268, 271, 282, 285, 300

Arithmetic - Fractions & Ratios: 182, 190, 201, 204, 233, 243, 255, 295

Arithmetic - Percents: 193, 199, 208, 222, 253, 266

Arithmetic - Pos/Neg & Odd/Even: 189, 196, 198, 213, 237

Arithmetic - Primes: 219, 270, 281

Algebra - Inequalities: 181, 188, 192, 200, 206, 215, 227, 231, 249, 252, 263, 276

Algebra - Linear Equations: 195, 218, 250

Algebra - Quadratics: 232, 239, 259, 265, 273, 279, 283

Algebra - Simultaneous Equations: 178, 202, 203, 217, 223, 230, 242, 269, 284

Geometry - Circles: 209, 216, 236, 256, 258, 278

Geometry - Coordinate: 191, 260, 290, 299

Geometry - Rectangles: 179, 185, 248

Geometry - Triangles: 261, 275, 291

Geometry - Other: 205, 207

Statistics - Averages: 194, 211, 228, 241, 244, 245

Statistics - Other: 225, 229, 235, 262, 294

Word Problems - Functions & Sequences: 197, 220, 264, 277, 280, 286, 287

Word Problems - Groups/Sets: 246, 274, 288, 293

Word Problems - Probability: 292

Word Problems - Revenue/Profit/Interest: 183, 186

Word Problems - Rate & Work: 187, 212, 289, 298