Tickets numbered 1 to 20 are mixed up and then a ticket is drawn at random. What is the probability that the ticket drawn has a number which is a multiple of 3 or 5?

1 2
2 5
8 15
9 20

Correct answer: 9 20

Solution

Multiples of 3 from 1 to 20 are 3, 6, 9, 12, 15, 18: 6 numbers. Multiples of 5 are 5, 10, 15, 20: 4 numbers. Since 15 is counted twice, total favorable outcomes = 6 + 4 - 1 = 9, so probability = 9/20.

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