If you break it up:
Three-eighths = 3/8
Sixteen times a number, we'll call the unknown number X, so 16X.
Twenty Four = 24
The sum of 16X and 24 = 16X + 24
We want 3/8 of that sum so we put the sum in parenthesis and we get:
Y = 3/8(16X + 24) where Y is the answer youd get if you knew what X was.