Standard deviation is a measure of how closely packed around the mean a particular set of data is.

Adding or subtracting to every number in the set does not affect that standard deviation, since that action changes the mean by the same amount as the numbers. 11, 12, 13, 14, and 15 are no more densely packed than 1, 2, 3, 4, and 5.

Multiplying or dividing, however, evenly scales the standard deviation by that same amount. It should be clear that 0.1, 0.2, 0.3, 0.4, and 0.5 are much more tightly packed that 1, 2, 3, 4, and 5. If you want to prove it to yourself mathematically, plug in some numbers to the actual equation (not needed for GMAT), found here: https://en.wikipedia.org/wiki/Standard_d ... Definition

So, 22.5 / 10 = 2.25. D.

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