Suppose that an office has 100 calculating machines. Some of them use electric power (E)
while others are manual (M) and some machines are well known (N) while others are used (U).
The table below gives numbers of machines in each category. A person enter the office picks
a machine at random and discovers that it is new. What is the probability that it is used with
E M Total
N 45 25 70
U 10 20 30
Total 55 45 100