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.

Then.

11/20 × x = 11

⇒ x = 20