Question

# A fair coin is tossed  a fixed number of times.  If the probability of getting 7 heads is equal to that getting 9 heads, then the probability of getting 3 heads is352123521472127214

Solution

## The correct option is A 35212Probability of r successes in n trials is equal to nCrprqn−r, where p is the probability of success in one trial and q is the probability of failure in one trial.  Let the coin be tossed n times. Here p and q are both 1/2  P(getting 7 heads)=nC7(12)n andP(getting 9 heads)=nC9(12)nGiven, P(7 heads)=P(9 heads) ⇒nC7=nC9⇒n=16∴P(3 heads)=16C3(12)16=35212.

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