Since selection is from 1 to 20 , number of sample space being selected from = 20 Probability is calculated by the formula below: We have to list all the numbers divisible by 3 (multiples of 3 ) from 1 to 20 to know how many numbers divisible by 3 in the sample Numbers divisible by 3 from 1 to 20=3,6,9,12,15 and 18 Hence 6 of the numbers are divisible by 3 Substite the number of numbers divisible by 3 and the total sample space into the probability formula Cancel out 2 goes into 6, 3 times and 20, 10 times