A math competition is scored by awarding 7 points for each correct response, 0 points for each incorrect response, and 2.5 points for each problem left unanswered. There are 20 problems in total. Ben attempts the first 18 problems and leaves the last 2 unanswered. How many of the first 18 problems must he solve correctly to achieve a score of at least 100 points?
12
13
14
15
16
