Tickets numbered from 1 to 20 are mixed up together and then a ticket is drawn at random. What is the probability that the ticket has a number which is a multiple of `3` or `7` ?

Explanation

Let .S. be the sample space associated with the given random experiment and .A. denote the évent "getting, a ticket bearing a number which is a multiple of 3 or .7 %.. Then, .S=1,2, ldots, 20. and .A=3,6,9,12,15,18,7, .14.
.therefore. Total number of elementary events .=20. .. Favourable number of elementary events .=8.
Hence, required probability. .=8/20=2/5.