First slide

Theorems of probability

Question

Three numbers are chosen from 1 to 20. The probability that they are not consecutive is

Easy

A

$\frac{186}{190}$
B

$\frac{187}{190}$
C

$\frac{188}{190}$
D

$\frac{189}{190}$

Solution

$P (T h r e e a r e n o t c o n \sec u t i v e)$ = $1 - P (T h r e e a r e c o n \sec u t i v e)$
= $1 - \frac{18_{c_{_{1}}}}{20_{c_{_{3}}}}$

= $\frac{187}{190}$

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