UPSC NDA II 2015 Mathematics Solved Paper

Let X denote the no. of coins showing 3 or more heads in a set of 7 coins
X follows binomial distribution with n=7
p = probability of getting a head in a single toss of a coin
⇒p=

1

2

; thus q=1−p=

1

2

∴ Probability of getting at least 3 heads
=P(x≥3)
=1−P(x<3)
=‌‌

128

128

−‌‌

29

128

=‌‌

99

128

∴ No. of throws=‌‌

99

128

×128=99