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In a stack of boards at a lumber yard, the 20th board counti

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In a stack of boards at a lumber yard, the 20th board counti

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In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack. How many boards are in the stack?

A. 38
B. 36
C. 35
D. 34
E. 32

OA D

Source: GMAT Prep

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BTGmoderatorDC wrote:
In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack. How many boards are in the stack?

A. 38
B. 36
C. 35
D. 34
E. 32

OA D

Source: GMAT Prep
If the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack, then the 19th board counting from the top of the stack is exactly the 16th board counting from the bottom of the stack. In other words, there are 18 boards above the 16th board counting from the bottom of the stack. Thus, there are 18 + 16 = 34 boards in the stack.

Answer: D

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Hi All,

Since the answers to the question are all numbers, we can use the answers 'against' the prompt and TEST THE ANSWERS. Here's how… Let's start with Answer B.

Answer B: 36 boards

IF the number of boards was 36, then the 20th board from the top would be the 17th board from bottom, which would be UNDER the 18th board from the bottom. This is NOT a match for what we were told. Elimate B. Now we know that the number of boards has to be SMALLER. Let's test D next.

Answer D: 34 boards

IF the number of boards was 34, then the 20th board from the top would be the 15th board from the bottom, which would be UNDER the 16th board from the bottom. This IS a match for what we were told, so this MUST be the answer.

Final Answer: D

GMAT assassins aren't born, they're made,
Rich

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Junior | Next Rank: 30 Posts Default Avatar
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Posted:
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Given,
the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack

implies board below the 20th board from top = 14th (16- itself - 1(the 20th) )
So, there are 14 boards below the 20th
So total boards = 20+14
= 34
Option D is correct

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GMAT/MBA Expert

Post
BTGmoderatorDC wrote:
In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack. How many boards are in the stack?

A. 38
B. 36
C. 35
D. 34
E. 32
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

19 = 16
20 = 15

14
13
12
11
10
9
8
7
6
5
4
3
2
1


In the rendering above:
The red values represent the number of boards when counting from the TOP.
The blue values represent the number of boards when counting from the BOTTOM.
The two middle rows with big numbers satisfy the constraint that RED BOARD 20 is directly below BLUE BOARD 16.

Total number of boards = (red boards 1-18) + (2 middle rows with big numbers) + (blue boards 1-14) = 18+2+14 = 34.

The correct answer is D.

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BTGmoderatorDC wrote:
In a stack of boards at a lumber yard, the 20th board counting from the top of the stack is immediately below the 16th board counting from the bottom of the stack. How many boards are in the stack?

A. 38
B. 36
C. 35
D. 34
E. 32
Let's start from the bottom and work our way up:

We'll start with the 16th board counting from the bottom of the stack

Right BELOW that 16th board, we have a board that we'll call Larry.
So, Larry is the 15th board counting from the bottom of the stack.
So, starting from the bottom and working our way up to Larry, we have 15 boards so far.

Larry is the 20th board counting from the TOP of the stack
In other words, there are 19 boards ABOVE Larry.
So, including those 19 boards, we now have 34 boards in TOTAL.

Answer: D

Cheers,
Brent

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