Discussion
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?
Answer: D
Count each set, then remove the overlap.
\[ \text{multiples of 3} = 6, \qquad \text{multiples of 5} = 4, \qquad \text{multiples of 15} = 1 \]
\[ 6 + 4 - 1 = 9 \quad\Longrightarrow\quad P = \frac{9}{20} \]
15 is a multiple of both and would otherwise be counted twice.
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