Set up 2 equations:
x + y = 21
Y- x = 13
Rewrite the last one as y = 13 + x
Now replace y in the first one:
X + (13+ x) = 21
Simplify:
2x + 13 = 21
Subtract 14 from both sides:
2x = 8
Divide both sides by 2:
X = 4
Now you have x solve for y:
Y = 13 + x = 13 + 4 = 17
The numbers are 4 and 17
