a number is selected at random from the first 40 natural numbers . what is the probability that the number is a multiple of either 2 or 21 ?
"number of multiples of 2 from 1 through 40 = 40 / 2 = 20 number of multiples of 21 from 1 through 40 = 40 / 21 = 1 number of multiples of 2 and 21 both from 1 through 40 = number of multiples of 21 * 2 ( = 42 ) = 0 total favourable cases = 20 + 1 - 0 = 21 probability = 21 / 40 answer : option a"
divide(add(floor(divide(40, 2)), divide(40, 21)), 40)