A combinatorics problem from NISER Nest 2017
An examination consists of 10 multiple choice questions, where each question has 4 options, only one of which is correct. In every question, a candidate earns 3 marks for choosing the correct option, and −1 for choosing a wrong option. Assume a candidate answers all questions by choosing exactly one option for each. Then the number of distinct combinations of answers which can earn the candidate a score from the set {15,16,17,18,19,20} is .
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1 solution
@Calvin Lin, @Chew-Seong Cheong could you please give a solution. I saw rhis in an exam and got a different one from that mentioned in the key (and here).