The probability that a teacher will give an unannounced test

The probability that a teacher will give an unannounced test during any class is 1/5. If a student is absent twice ,then probability that misses atleast one test is ?

Solution

Given the probability of teacher will give an unannounced test during any class is 1/5.

The probability of not missing test 1-1/5=4/5

probability of missing test in only one day 2(4/25)=8/25

probability of missing test on both the day (1/5)*(1/5)=1/25

probability that misses atleast one test is (8/25)+(1/25)=9/25

probability that misses atleast one test is 9/25