A-LevelMathematicsProbability and statistics9709/51 · Oct/Nov 2021 · Q14 Marks
Two fair coins are thrown at the same time. The random variable X is the number of throws of the two coins required to obtain two tails at the same time.
- (a)Find the probability that two tails are obtained for the first time on the 7th throw.[2]
- (b)Find the probability that it takes more than 9 throws to obtain two tails for the first time.[2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.
- [M1] (1-p)^6 p, 0<p<1
- [A1] 0.0445, 729/16384
- [M1] (3/4)^9 or p^n, 0<p<1,n=8,9,10
- [A1] 0.0751, 19683/262144
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