A pair of fair coins is thrown repeatedly until a pair of tails is obtained. The random variable $$$X$$$ denotes the number of throws required to obtain a pair of tails. Find the probability that exactly 3 throws are required to obtain a pair of tails. ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................ ........................................................................................................................................................

Mathematics

IGCSE&ALevel

CAIE

Exam No：9709_s20_qp_53 Year：2020 Question No：5(b)

Answer:

$\frac{9}{64} \quad(=0.141)$

Knowledge points:

5.4.2 use formulae for probabilities for the binomial and geometric distributions, and recognise practical situations where these distributions are suitable models (Including the notations B(n,p) and Geo(p). Geo(p) denotes the distribution in which for r = 1,2,3,...)

Answer:

Knowledge points:

Solution:

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