Eliminate U and solve for J, U=JW, N=UW

First, rearrange U=JW for J. You should get J=U/W.

Next, solve for U in N=UW. You should get U=N/W

Now, substitute U=N/W for U in J=U/W. Your equation should now be J = (N/W) / W.

Since, this is bad form, multiply 1/W by the reciprocal of N/W. Your equation should now be J=W / (WN).

Since, W/W=1, then all that is left is 1/N.

Therefore J=1/N.