(18 + 24 + 32 + x)/4 >/= 24.
(we use the inequality because her average must be AT LEAST 24 points).
Finding for x
(74 + x) >/= 24 x 4 ==> 74 + x >/= 96
x = 22.
So, Sophia must score at least 22 points in her last game so that her four-game average is at least 24 points.