Deci- means 1/10 or .1 of a unit.
$.07. You do not write $.7 because that would mean 70 cents and should be written with a 0 at the end anyways. You do not write any dollar amount with a cents sign at the end, because the dollar sign replaces it. You write $.07 because 7 cents means 7/100 (7 one hundredths) so the seven must be written in the hundredths spot. Remember, the hundredth spot is the second number after a decimal.
Alright, 14 oz. cup of coffee contains 20% milk. If you add 7 ounces to that mixture, your answer would be:
14 oz = 0.2 milk
+7 oz. = 28
28/14 = 2
0.2*2 = 0.4, written as a percent will be 40%
40% milk.
The answer to your question is b
