Is the question r=5n and 5z=r? If so, we replace r in the first equation with 5z from the second equation and we get 5z=5n. So n=z. If the question is really r=5n+5z=r, the question doesn't really make sense because there are two equals signs, and the second one only serves to reinforce the first one and the equation is just r=5n+5z, so 5n=r-5z and n=(1/5)(r-5z).