Probability that a random integer between 1 and 100 is divisible by 3 or 5?

Probability that a random integer between 1 and 100 is divisible by 3 or 5?

Approach: Count 3s, count 5s, subtract 15s.

47/100. 33 + 20 - 6 = 47.

Follow-up: How many are divisible by neither?

Key concepts: inclusion-exclusion.