A heap of rubbish in the shape of a cube is being compacted into a smaller cube. given that the volume decreases at a rate of 4 cubic meters per minute, find the rate of change of an edge, in meters per minute, of the cube when the volume is exactly 125 cubic meters.

Working Formula:

V = s^3

Given:

dV/dt = -4 cubic meters per minute
V = 125 cubic meters

Required: ds/dt (rate of change of edge per minute)  at V = 125 m^3

Solution:

Differentiate, equation below 
V = s^3
dV/dt = 3*s^2 (ds/dt)
-4 = 3*s^2 (ds/dt)       
ds/dt = -1.33/s^2  -----------> eq. (1)

V = s^3
(125)^0.33 = (s^3)^0.33
s = 5                   ------------> eq. (2)

Substitute eq. (2) to eq. (1), we get

ds/dt = -1.33/(5)^2 = -0.053 meters per minute

ANSWER: -0.053 meters per minute