Out of 20 tickets numbered from l to 20, one can be chosen in 20 ways. So, total number of elementary events associated with the given random experiment is 20. Out of 20 tickets numbered from l to 20, tickets bearing numbers which are multiple of 3 or 7 bear numbers 3, 6, 7, 9, 12, 14, 15 and 18.
Therefore,
Favourable numbers of elementry events = 8
Hence, required probability `=(8)/(20)=(2)/(5)`
