Question 847492
x/20 + y/30 = 3 
x/40 + y/15 = 3

multiply by 120 just to make everything nice.

6x + 4y = 360
3x + 8y = 360

Multiply the top equation by 2.

12x + 8y = 720
3x + 8y = 360
------------------
9x = 360

x = 40

Plug back in x = 40 where convenient

x/20 + y/30 = 3

40/20 + y/30 = 3

2 + y/30  = 3

y/30 = 1

y = 30


So x = 40, y = 30