Question

Three numbers are chosen from 1 to 20. Find the probability that they are consecutive.

 

Answer 1

\[\text{ Total possible outcomes } =^{20} C_3 \]
\[\text{ Consecutive numbers chosen } = (1, 2, 3), (2, 3, 4) . . . (18, 19, 20)\]
\[\text{ So, there are 18 favouable cases } .\]
\[P\left( A \right) = \frac{18}{{C^{20}}_3}\]