Question

let xy=5 and let dy/dt=5 find dx/dt when x=4
I solved for y first and got 5/4:
xy=5
y=5/x => 5/4
(x)(dy/dt) + (5/4)(dx/dt) = 0
(4)(5) + (5/4)(dx/dt) = 0 => 20 + (5/4)(dx/dt) = 0
dx/dt = -20/(5/4)
Webworks is saying it is wrong though?