Ian Stewart wrote:I'd likely do the question algebraically, as was done above, but there are certainly other options, some quite fast. If we are careful to choose extremely simple numbers, for x and y, we can determine which answer choice describes the relationship between x and y. Say our investment is worth $100 to begin with, then doubles in value to $200, then drops back to $100. Then we have a 100% increase followed by a 50% decrease; that is, x = 100 and y = 50. We can now plug x = 100 into each answer choice until we find one that is equal to 50; only E is remotely close.

Note that if, by bad luck, two different answer choices both gave us 50, we would need to try a different set of numbers to work out which of those two choices was correct - one reason why it can be preferable just to do things algebraically.