Joe is at most 20 and Bill would be at most 27 because it has to be less than 49 combined
so I think u you divide something and you times and then you multiply it
Instantaneous rate of change = S'(r) = 8πr
S'(8) = 8π(8) = 64π
Therefore, the instantaneoud rate of change of the <span>surface area with respect to the radius r at r = 8</span> is 64π
The work is on the picture attached, but a = 1
Hope this helps!!