Working together at their respective constant rates, A and B can fill an empty tank in 1/2 hr. What is the constant rate of B?
1) Constant rate of A = 25 liters/min
2) Tank capacity = 1200 liters
Yes, (C) is correct. Here is the most efficient way to think about this question.
Step 1) Recognize this is a typical distance = rate * time question. Technically they use the term "capacity"--but really the concept is the same.
Step 2) Recognize there are two components that combine into a total so it's a little bit trickier. This means we need to use one of our "special" formulas:
a) rate (A) + rate (B) = rate (total)
The rates must add up!
or
b) 1/time(A) + 1/time(B) = 1/time(total)
Pick whichever one works best for you. Let's say we choose (a)
Step 3) rate (A) + rate (B) = rate (total)
We know [rate (A)].
We're supposed to find [rate (B)].
Do we know the last one: [rate (total)]?? If we do, then we have enough information.
No we don't know the combined rate.
But do we know anything else about the total that might help us find the combined rate?
We know the combined total time was 1/2 hr.
And we know [rate = distance / time]. We know time, so all we need is the distance--then we'll find the total rate that we need. But as it stands now, there's not enough info in only statement 1 so the answer is not (A)
Step 4) Now what about if we knew statement 2 but did not know statement 1? Same thing. That means the answer is not (B)
In this equation: rate (A) + rate (B) = rate (total)
We need 2 terms in order to find the 3rd missing term.
Step 5) We need both rate (A) AND rate(total). In order to find rate(total) we'll need the distance ("capacity")--which is statement 2.
Therefore, we need both statements (1) and (2)--so the answer is (C).
Notice we did not have to do any calculations. In fact, calculations slow you down. You should be able to answer this question in less than 2 minutes.
