Problem Solving

The two compound interest questions in your post are not even close to being realistic GMAT questions. Where are they from? The first requires a calculator to solve, and the second doesn't make any mathematical sense. Anyone preparing for the GMAT should ignore them.

Only the second question even resembles a real GMAT question, and for that question to be solvable, you need to be told that the distribution of data is 'symmetric'.

vishalchaudhury wrote:
Hi Ian,

Does the 3rd Q not make sense because the interest per month will be same till compounding is done? Can you please help me understand the 2nd question?

Yes, it's not clear what they mean by 'interest in the first month' if the interest is compounded quarterly, which normally means that interest is applied once every three months. It's not even clear why the compounding should be relevant if we only look at a two month period. The question just doesn't make any sense.

vishalchaudhury wrote: Q. The mean of a list of numbers is m and the deviation (not sure here) is n. It is known that 68% of the numbers are within m and n, what is the percentage of the numbers that are less (or more) than m+n?

I don't know why you say 'not sure here' when reprinting the question - where are you getting these questions from? It's obviously impossible to answer a question if you don't even know what the question is asking.

The question ought to say that the set of data is distributed 'symmetrically'; otherwise it cannot be answered. If 68% of values are then within one standard deviation of the mean (so between m-n and m+n), then 32% (the rest) of values are more than one standard deviation away from the mean (so either greater than m+n or less than m-n). If the distribution is symmetric, half of these will be greater than m+n, so 16% of values will be greater than m+n, and thus 84% will be less than m+n.

There's a technicality with the wording that I've ignored here, since if the question were properly worded, you'd be told to ignore it: the question needs to rule out the possibility that some values are exactly equal to m+n, which in a small set might have some effect on the answer.