0.5x+0.20y=3.15 what are the values of x and y if x+y=9.0

Use elimination or substitution to solve.

I'll use substitution here because the second equation can easily be put in terms of one variable.

x+y=9.0

y=9.0-x

substitute 9.0-x into the other equation for y

0.5x + 0.2 (9.0-x) = 3.15

0.5x + 1.8 - 0.2x = 3.15

0.3x + 1.8 = 3.15

0.3x = 3.15 - 1.8

0.3x = 1.35

x = 1.35/0.3

x = 4.5

then if x = 4.5

4.5 + y = 9.0

y = 9.0 - 4.5

y = 4.5