# A box contains tickets numbered from 1 to 20. Three tickets are drawn from the box with replacement. The probability that the largest number on the tickets is 7 is

Question : A box contains tickets numbered from 1 to 20. Three tickets are drawn from the box with replacement. The probability that the largest number on the tickets is 7 is

(A) 2/19

(B) 7/20

(C) $1-\left( \dfrac{7}{20}\right) ^{3}$

(D) none of these

Then, $P\left( X\leq 7\right) =\left( \dfrac{7}{20}\right) ^{3}$ and $P\left( X\leq 6\right) =\left( \dfrac{6}{20}\right) ^{3}$. Then the required probability is,
$p( x= 7) =\left( \dfrac{7}{10}\right) ^{3}-\left( \dfrac{6}{20}\right) ^{3}$.