Let the coin is tossed ' n ' times. Also, let x denote number of times head occurs. According to the question, P(X=7)=P(X=9) Using formula P(X=r)=nCtptqn−r where, p= probability of getting head in tossing a coin =1∕2 and q=1−p=1−1∕2=1∕2 ∴P(X=7)=P(X=9) ⇒nC7(
1
2
)7(
1
2
)n−7=nC9(
1
2
)9(
1
2
)n−9 ⇒nC7(
1
2
)n=nC9(
1
2
)n ⇒nC7=nC9 ⇒n=7+9=16[∵nCr=nCq only when n=r+q] ∴P(X=2)=16C2(
