The correct option (a) 8
Explanation:
Let n be the number of bomb required and x be number of bomb that hit the bridge.
x follows binomial distribution with parameter n and r = (1/2).
P(x ≥ 2) > 0.9
⇒ 1 – P(x < 2) > 0.9
⇒ P(x < 2) < 0.1
∴ P(x = 0) + P(x = 1) < 0.1
∴ (1/2)n + n ∙ (1/2)n < 0.1
∴ (1/2)n [n + 1] < (1/10)
∴ 10n + 10 < 2n
from options, n = 8 satisfies above inequality.