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An executive in a company makes on an average 5 telephone calls per hour at a cost of rupees 2 per cell. The probability that in any hour the cost of the calls  exceeds  a sum of  rupees 4 is 

Answer:

Explanation:

 The number of telephone calls per hour is a random variable mean =5

 The required  probability is given by

  $P(r>2)=1-P(r\leq2)=1-\sum_{r=0}^2\frac{e^{-5}.5^{r}}{r!}$

  $=1-\left[\frac{5^{0}}{0!}+\frac{5^{1}}{1!}+\frac{5^{2}}{2!}\right]\frac{1}{e^{5}}=1-\frac{37}{2e^{5}}=\frac{2e^{5}-37}{2e^{5}}$