Question 759888
Volume = length * width * height = 20*12*10 = 2400 cm^3
now we want the following
(20-x)*(12-x)*(10-x) = 768
multiplying and combining like terms we have
-x^3 +42x^2 -560x +1632 = 0
then take the first derivative f'
f' = -3x^2 +84x -560
and x = (-b +or- sqrt(b^2 -3ac)) / 3a    note if b^2-3ac > 0 the cubic has a min and max
this is the case for this problem and 4 is the min