Total number of balls `= (9 + 12) = 21.`
Let S be the sample space. Then,
n(S) = number of ways of selecting 1 ball out of 21 = 21.
Let E be the event of drawing a red ball. Then,
n(E) = number of ways of selecting 1 red ball out of 9 = 9.
`therefore` P(getting a red ball) `= P(E) = (n(E))/(n(S)) = 9/21 = 3/7.`