ITPEC FE Morning April 2020 Question 2

Probability — complement method — find probability of "at least K" outcomes.

"At least 2 heads in 5 flips" → use complement:

• •Total outcomes: 2^5 = 32
• •P(0 heads) = C(5,0) = 1/32
• •P(1 head) = C(5,1) = 5/32
• •P(≥2) = 1 − 6/32 = `26/32 = 0.8125`

Why not others:
- (a) 0.78125 — equals 25/32, incorrect complement calculation
- (c) 0.84375 — equals 27/32, miscounted one case
- (d) 0.875 — equals 28/32, subtracted only P(0) and forgot P(1)

Key rule: For "at least K" problems, compute 1 − P(less than K) using the complement method.

AI-generated — may contain errors