Bill is thinking of 2 numbers. they have a sum of 28 and a difference of 8. what are the 2 numbers?

Let the two numbers be a and b respectively.
a + b = 28 1.
a - b = 8 2.
Solve those equations simultaneously.

1.-2.
a - a + b - - b = 28 - 8
2b = 20
b = 10

Replace b in equation 1 or 2
a + b = 28
a + 10 = 28
a = (28 - 10) = 18

sum = 10+18 = 28
difference = 18-10 = 8

Hope it helped!