Question

A box contains 20 balls bearing numbers 1, 2, 3, 4, ..., 20. A ball is drawn at random from the box. What is the probability that the number on the ball is not divisible by 10?

Answer 1

Given: A box contains the numbers 1, 2, ..., 20 and one ball is drawn at random.

Step-wise calculation:

1. Total possible outcomes = 20 (numbers 1 to 20).

2. Numbers divisible by 10 in this range: 10 and 20 → 2 numbers.

3. Number of favourable outcomes (not divisible by 10)

= 20 – 2

= 18

4. Probability = `"Favourable outcomes"/"Total outcomes"` 

= `18/20`

= `9/10`

= 0.9

= 90%

The probability that the number on the drawn ball is not divisible by 10 is `9/10`.