Max@Math Revolution wrote:[GMAT math practice question]

A and B are positive integers. G is the greatest common divisor of A and B, and L is the least common multiple of A and B. What is the value of A + B?

1) G/A + G/B = 7/10
2) L=70

Rule:
For any two positive integers x and y, the product of x and y is equal to the product of their greatest common factor and their least common multiple.
xy = (GCF)(LCM)

A and B are positive integers. G is the greatest common divisor of A and B, and L is the least common multiple of A and B.
Applying the rule above, we get:
AB = GL
L = (AB)/G

Statement 1:
G/A + G/B = 7/10
(BG + AG)/(AB)= 7/10
(B+A) * G/AB = 7/10
A+B = 7/10 * (AB)/G

Since L = (AB)/G, we get:
A+B = (7/10)L

If L=10, then A+B = (7/10)(10) = 7
If L=70, then A+B = (7/10)(70) = 49
Since A+B can be different values, INSUFFICIENT.

Statement 2:
Case 1: A=1 and B=70, with the result that LCM(1, 70) = 70
In this case, A+B = 1+70 = 71
Case 2: A=7 and B=10, with the result that LCM(7, 10) = 70
In this case, A+B = 7+10 = 17
Since A+B can be different values, INSUFFICIENT.

Statements combined:
Substituting L=70 into A+B = (7/10)L, we get:
A+B = (7/10)(70) = 49
SUFFICIENT.

The correct answer is C.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 2 variables (A and B) and 0 equations, C is most likely to be the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Suppose A=aG and B=bG for some integers a, b and G, where a and b are relatively prime.

Then
G/A + G/B = 7/10
=> G/(aG) + G/(bG) = 7/10, since A=aG and B = bG
=> bG/(abG) + aG/(abG) = 7/10, taking a common denominator
=> (aG+bG)/(abG) = 7/10
=> (A+B)/L = 7/10
=> (A+B)/70 = 7/10, since L = 70
=> A+B = 49
Since both conditions together yield a unique solution, they are sufficient.

Since this question is an integer question (one of the key question areas), CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B.

Condition 1)
If A = 2 and B = 5, then A + B = 7.
If A = 6 and B = 15, then A + B = 21.
Since condition 1) doesn't yield a unique solution, it is not sufficient.

Condition 2)
If A = 14 and B = 35, then A + B = 49.
If A = 2 and B = 35, then A + B = 37.
Since condition 2) doesn't yield a unique solution, it is not sufficient.

Therefore, C is the answer.
Answer: C

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.