Question 4842
I think you mean solve by using the elimination method...

  x   -y = 100 
.2x +.6y = 150

Multiply the second equation by -5

 x-  y = 100
-x+-3y =-750

Add the two equations to get

-4y=-650

Divide both sides by -4
y=162.5

Substitute 325 into first equation to get
x-162.5=100
Add 162.5 to both sides
x=262.5

check your answer with the second equation:
.2(262.5)+.6(162.5)=150
52.5+97.5=150  (True statement)

The solution is (262.5,162.5)