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?
5-Day Free Trial
5-day free, full-access trial TTP
Available with Beat the GMAT members only code
MORE DETAILS
What is the value of (1/b) + (3/d)
This topic has expert replies
GMAT/MBA Expert
-
[email protected]
- Elite Legendary Member
- Posts: 10392
- Joined: Sun Jun 23, 2013 6:38 pm
- Location: Palo Alto, CA
- Thanked: 2867 times
- Followed by:511 members
- GMAT Score:800
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
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
- GMAT Instructor
- Posts: 2630
- Joined: Wed Sep 12, 2012 3:32 pm
- Location: East Bay all the way
- Thanked: 625 times
- Followed by:119 members
- GMAT Score:780
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.
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.
