That's not correct... the multiplied out equation is not sufficient to determine the answer... For example, from the multiplied equation, you could have something like:
y=1, j=2, u=3, c=3, k=4 and that would work without yj being equal to 0.
The answer is jy=0 though...
I just looked at the three equations (including c<k) and saw that one of the following two things had to happen:
Either [(u=c and j=0) or (u=k and y=0)] or (j=0 or y=0)
No matter which is the case, there's always a requirement for either j=0 or y=0. So we know that jy has to equal 0.
netigen wrote:to get yj (u - c) (u - k) = 0
we know that c<k so the above is only possible when yj = 0