chaitanya.bhansali wrote:Tap 1 takes 1.5 hours to fill a certain tank with water. Tap 2 fills the same tank with water at a rate X times as fast as tap 1. Tap 1 and tap 2 together take 15 minutes to fill the same tank with water. Find the value of X.
A) 1/9
B)1/5
C) 5
D) 9
E) 45
Given that the answer choices are so spread apart, we can skip the formal approaches and apply some common sense.
Aside: The importance of ALWAYS checking the answer choices BEFORE choosing a specific approach is covered in our free video on General GMAT Math Strategies: https://www.gmatprepnow.com/module/gener ... es?id=1111
Okay, let's say that YOU are tap 1, and it takes you 90 minutes to complete some job (e.g., fill a tank). You're looking for someone to help you so that, TOGETHER, you can complete the job in
15 minutes.
Answer choice A) Joe offers to help, but he's 1/9 as fast as you are.
Yeesh, this guy is useless!
NOTE: if Joe worked at the SAME speed as you, then you'd be able to finish in half the time. In other words, the two of you would complete the job in 45 minutes. However, since Joe is A LOT slower than you, it will take
LONGER than 45 minutes to complete the job. ELIMINATE A
Answer choice B) Al offers to help, but he's 1/5 as fast as you are.
Using the same logic that we used above, we can conclude that since Al is A LOT slower than you, it will take
LONGER than 45 minutes to complete the job. ELIMINATE B
Answer choice E) Bob offers to help, and he's 45 times as fast as you.
NOTE: Bob works so fast that your contribution makes little difference. In fact, since Bob is 45 times as fast as you, he could work ALONE and finish the job in 1/45 of the time. In other words, working ALONE, Bob would complete the job in 2 minutes. Of course, with YOUR HELP, the two of you would take
LESS than 2 minutes to complete the job. ELIMINATE E
Answer choice D) Gary offers to help, and he's 9 times as fast as you.
Since Gary is 9 times as fast as you, he could work ALONE and finish the job in 1/9 of the time. In other words, working ALONE, Gary would complete the job in 10 minutes. Of course, with YOUR HELP, the two of you would take
LESS than 10 minutes to complete the job. ELIMINATE D
Answer choice C) Hal offers to help, and he's 5 times as fast as you.
Since Hal is 5 times as fast as you, he could work ALONE and finish the job in 1/5 of the time. In other words, working ALONE, Hal would complete the job in 18 minutes. Of course, with YOUR HELP, the two of you would take
a little LESS than 18 minutes to complete the job. Perfect -
15 minutes is a little less than 18 minutes.
Answer:
C
Cheers,
Brent
