under certain conditions, the thrust T of a propeller varies jointly as the fourth power of it's diameter, d, and the square of the number, n, of revolutions per second. What happens to the thrust if n is doubled and d is halved?

Answer 1

T = kd^4n^2

if we replace n by 2n and d by d/2, we have

T' = k(d/2)^4(2n)^2 = kd^4n^2/4 = T/4