A woman sold 100 oranges at $12.10, some at the rate of 3

This can be solved like a classical mixture problem but numbers are awkward to deal with.

It's easier to just look at the answer choices. You know that a multiple of 3 oranges has to be sold at the first rate, and a multiple of 7 at the second rate. You simple subtract the answer choices for the first rate from 100 and check whether the remainder (i.e. the number of oranges sold at the second rate) is a multiple of 7.

100 - 45 = 55 => not a multiple of 7 so exclude
100 - 21 = 79 => not a multiple of 7 so exclude
100 -9 = 91 => a multiple of 7 so keep
100 - 15 = 85 => not a multiple of 7 so exclude
100 - 12 = 88 => not a multiple of 7 so exclude

Hence, answer choice 9 is correct.