Question

What is the probability of a 20 year flood occurring four times in the next 20 years ?

• A. 0.0133
• B. 0.133
• C. 0.00133
• D. 0.000133

Answer 1

The correct option is A 0.0133

20 years flood means flood of return period 20 years

$\therefore \text{Probability of exceeding,}p=\frac{1}{20}$

Probability of occurrence four time in next 20 years,

$P_{4.20}{=}^{20}{C}_4\times {\left(\frac{1}{20}\right)}^4\times {\left(\frac{19}{20}\right)}^{16}$

$=\frac{20!}{4!\times 16!}{\left(\frac{1}{20}\right)}^4\times {\left(\frac{19}{20}\right)}^{16}$

= 0.01332