Tickets numbered 1 to 20 are mixed. What is the probability the drawn ticket is a multiple of 3 or 5?

Given data

• Sample space size = 20.
• Multiples of 3: 3,6,9,12,15,18 (6 numbers).
• Multiples of 5: 5,10,15,20 (4 numbers).

Concept/ApproachApply inclusion–exclusion: count(3 ∪ 5) = count(3) + count(5) − count(15).

Step-by-step calculationCommon multiples (15): 1 (i.e., 15)Favourable = 6 + 4 − 1 = 9Probability = 9/20 = 9/20

Verification/AlternativeExplicit list: {3,5,6,9,10,12,15,18,20} (9 numbers).

Common pitfallsDouble-counting 15 or forgetting it entirely.

Final Answer9/20