The probability of selecting a blue marble at random from a jar that contains only blue, black and green marbles is 1/5. The probability of selecting a black marble at random from the same jar is 1/4 . If the jar contains 11 green marbles, find the total number of marbles in the jar.
P (blue marble) = 1/5
P (black marble) = 1/4
∴ P (green marble) = 1 – (1/5 + 1/4)
P (green marble) = 11/20
Let total number of marbles be x.
11/20 × x = 11
⇒ x = 20