The cost of buying x units of a certain product is given by the

Hi petu,

This question is based around a relatively 'rare' issue in DS: 'redundant' or unnecessary calculations.

We're told that the cost of buying X units is given by the formula Cost = A + (X + H)B where A, B and H are unknown constants. We're asked for the DIFFERENCE in the cost of buying 50 units vs. buying 25 units.

Before going to the Facts, I'm going to set up an equation based on what we're asked to calculate.

Cost of 50 units = A + (50+H)B
Cost of 25 units = A + (25+H)B

You should notice that the equations are almost identical. We're asked for the DIFFERENCE in costs, so we have to subtract the second equation from the first equation....

[A + (50+H)B] - [A + (25+H)B]

We can simplify this expression and 'cancel out' pieces....

First, the "A's" cancel out...leaving us with....

50B + HB - 25B - HB

Next, let's combine like terms.....this leaves us with....

25B

So the question is REALLY asking "what is the value of 25B?"

Fact 1: H = 50

This does NOT tell us the value of B, so we cannot answer the question.
Fact 1 is INSUFFICIENT

Fact 2: B = 60

This DOES tell us the value of B, so the answer to the question is (25)(60) = 1500....although we didn't really have to do that work.
Fact 2 is SUFFICIENT

Final Answer:

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