M+n=100, m-n=30 what is m and what is n

 First, you solve for m by adding the equations.
m+n=100
+ m-n=30

2m=130
m=65

Now plug in 65 for m into one of the original equations
65+n=100
n=35

You can double check by plugging them both in:
65+35=100 and 65-35=30

Hope that helps.