It's a simultaneous equation: Steps: 1.Number the equations..
a+b=77 -1 a-b=13 -2
2. Choose what variable you want to use. In this case I would use the "b". Since the signs in front of the "b's" are different, add the two equations together
a + b = 77 + + + a (-b) = 13
Which gives; 2a = 90
Then solve to find a: 2a=90 a= 90/2 a=45
3.Then plug the "a" value into any of the original equations to find the "b" value. I would use equation 1 since the all the variables are positive.
