Refer the image
Equations for permutations and combinations
The Attempt at a Solution
Let x be the no. of questions that turned out to be correct. So total score will be 3x-(10-x)=4x-10.
The value of this expression must be from the given set and since x is an integer, the only no. satisfying the condition from the set is 18. (4(7)-10=18).
Hence 7 questions were correct and 3 incorrect. So, no. of ways of choosing 7 questions from 10 would be 10C7 = 120. Isn't this what has been asked in the question or is it something else?
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