WE have 10 questions, the answer is either right or wrong whatever the number of choices. Then P(right) = 1/2 and Q(wrong) =1/2 Moreover as per the given of the problem, he may get at least 7 right answers, that means he can get 7 OR 8 OR 9 OR 10 right answers: Here-below the binomial probability distribution:
a) for 7 exactly right answers : ¹⁰C₇ (1/2)⁷(1/2)³ = 0.117187
b) for 8 exactly right answers : ¹⁰C₈ (1/2)⁸(1/2)² = 0.043945
c) for 9 exactly right answers : ¹⁰C₉ (1/2)⁹(1/2)¹ = 0.009656
d) for 10 exactly right answers : ¹⁰C₁₀ (1/2)¹⁰(1/2)⁰ = 0.000488 ------------- Σ (PROBABILITY) = 0.171276
