Question

A wallet contains a Php 20.00 bill, a Php 50.00 bill, and a Php 100.00 bill. A bill is taken out from the wallet and then replaced with a bill of the same amount that was taken away. If a bill is taken out from the wallet, what is the probability that the sum of the bills is

A. Php 70.00

B. Not Php 70.00

C. More than Php 40.00

Answer 1

According to the question:

The wallet contains 1 20 Php bill

1 50 Php bill

1 100 Php bill

(A). The amount is taken with replacement

So, the probability of getting 70 Php bill = $2\times {1 \over 3 }\times {1 \over 3}={2\over 9}$

(B). P(Not 70)= $1 - \dfrac{2}{9}= \dfrac{7}{9}$

(C). P(> 40)= P(70) + P(120) + P(150)= $2\times ({1 \over 3 }\times {1 \over 3})+2\times({1 \over 3 }\times {1 \over 3})+2\times({1 \over 3 }\times {1 \over 3})= \dfrac{2}{3}$