Answer:
87 miles per second
Explanation:
We can represent the speed at which Brock throws the baseball in a ratio of miles to seconds:
miles : seconds
29 : 1/3
To get the left side of the ratio to be the number of miles he can throw in one second, we have to multiply both sides by a number such that the right side is 1. Since the right side is currently 1/3, that number is 3:
miles : seconds
(29 : 1/3) · 3
87 : 1
So, Brock can throw a baseball 87 miles per second.
Sidenote
It's physically impossible for a human to throw anything at 87 miles per second. Actually, it might even be impossible for anything to move that fast through the Earth's atmosphere, as the air resistance would lead to so much heat that the object would most likely disintegrate. For reference, the speed of sound is about 0.213 miles per second, so throwing a baseball at the speed in this problem's answer would be about Mach 408.