Cost of assembling x wallclocks is `( x^3/3 - 40x^2)` and labour charges are 500x. Find the number of wall clocks to be manufactured for which average cost and marginal cost attain their respective minimum.
Total cost = Assembling cost + Labour cost
Total cost = `x^3/3 - 40x^2 + 500x`
C(x) = `x^3/3 - 40x^2 + 500x`
`C_M = (dc)/(dx)`
`C_M = x^2 - 80x + 500`
`(dC_x)/dx = 2x - 80`
`C_M "is minimum if "(dC_M)/dx = 0 and (d^2C_M)/(dx^2) > 0`
∴ `(d^2C_M)/dx = 0`
∴ 2x - 80 = 0
∴ x = 40
`(d^2C_M)/(dx^2) = 2`
`(d^2C_M)/(dx^2)` > 0 at x = 40
∴ Marginal cost is minimum at x = 40.