Answer
$P(7\leq x\leq9)$ = 0.6385
Interpretation: The probability that at 7 to 9 (inclusive) adult smokers started smoking before the age of 21 is 0.6385. If we carried out 100 trials of this experiment, we expect that approximately 64 trials will result in 7 to 9 (inclusive) adult smokers who started smoking before the age of 21.
Work Step by Step
$P(7\leq x\leq9)$
$= P(7) + P(8) + P(9)$
Using the binomial formula ${n}\choose{x}$ $\cdot$ $p^xq^{n-x}$, we have:
P(7) = ${10}\choose{7}$ $\cdot$ $0.90^7\cdot0.10^{10-7}$ = 0.05740
P(8) = ${10}\choose{8}$ $\cdot$ $0.90^8\cdot0.10^{10-8}$ = 0.1937
P(9) = ${10}\choose{9}$ $\cdot$ $0.90^9\cdot0.10^{10-9}$ = 0.3874
$P(7\leq x\leq9)$
= 0.05740 + 0.1937 + 0.3874
= 0.6385
