The sum of three positive numbers is 60. The first plus twice the second plus three times the third

add up to 120. Find the numbers which maximized the product all the three numbers.

2 answers

x+y+z = 60
x+2y+3z = 120

x-z = 0
so x=z
and we have
f(x) = x^2(60-2x)
f'(x) = 120x - 6x^2 = 6x(20-x)
so the numbers are 20,20,20

as usual, maximum volume of a prism (for a given surface area) is when it is a cube.
why does x-z became equal to zero?