Maths problem: i don't know how to do (c), it's so difficult, thanks?

(a)
P(red) = x/48 = 1/16, so x = 48/16 = 3.
P(yellow) = y/48 = 1/12, so y = 48/12 = 4.

(b)
The number of black balls is
48 - x - y
= 48 - 3 - 4
= 41

Therefore, P(black) = 41/48.

(c)
Let the required number be n.

After taking out n black balls, there are (48 - n) balls in total where (41 - n) are black.

Therefore,
P(black) = (41 - n)/(48 - n) = 4/5
5(41 - n) = 4(48 - n)
205 - 5n = 192 - 4n
5n - 4n = 205 - 192
n = 13

Thus, 13 black balls should be taken.