What is the value of (1/b) + (3/d)

[ssyohee]

1) The greatest common divisor of b and d is 6
2) The least common multiple of b and d is 12

The answer is E

Can someone explain it?

[[email protected]]

Hi ssyohee,

This question can be solved by TESTing VALUES. There's also a great time-saving 'shortcut' in that we are never told which variable represents which number (so we an 'interchange' them).

We're asked for the value of (1/B) + (3/D).

1) The greatest common divisor of B and D is 6

IF....
B=6
D=12
Then the answer to the question is 1/6 + 3/12 = 5/12

IF....
B=12
D=6
Then the answer to the question is 1/12 + 3/6 = 7/12
Fact 1 is INSUFFICIENT

2) The least common multiple of b and d is 12

The SAME two TESTs that 'fit' Fact 1 also fit Fact 2 (and create the two different answers shown above.
Fact 2 is INSUFFICIENT

Combined, we have the same two 'overlapping' answers that are different.
Combined, INSUFFICIENT

Final Answer: E

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

[Matt@VeritasPrep]

Let's set it up first.

1/b + 3/d =

d/db + 3b/db =

(d + 3b)/db

Now we'll take our statements:

S1: We could have b = d = 6, or b = 12, d = 6, so we'll get different results; NOT SUFFICIENT.

S2: We could have b = d = 12, or b = 1, d = 12, so again we'll get different results; NOT SUFFICIENT.

Together, let's use a great formula:

LCM of x,y * GCF of x,y = x * y

So we have

12 * 6 = b * d, or db = 72

This gives us our denominator, so we have

(d + 3b)/db = (d + 3b)/72

But we still don't know d and b! We could have d = 12, b = 6, or d = 6, b = 12. So our answer could be 30/72 or 42/72; NOT SUFFICIENT.