IIT-JEE 1985Question 1
In a multiple-choice question there are four alternative answers, of which one or more are correct. A candidate will get marks in the question only if he ticks the correct answers. The candidate decides to tick the answers at random. If he is allowed up to three chances to answer the question, find the probability that he will get marks in the question.
Probability
Answer$\dfrac15$
Solution
There are $2^4-1=15$ nonempty choices and exactly one is correct. With three distinct random attempts, probability $=\frac3{15}=\frac15$.