We are looking for the value of x if we know the value of y=0.8 and the equasion is: 4x + 8y = 40. First we separate what we don't know on the left side and what we do know on the right: 4x = 40 - 8y. we know y=0.8 so we put that in the equation and we get: 4x = 40 - 8*0.8. Now we have: 4x = 40 - 6.4=33.6. Finally, we divide the left side and the right side by 4 and we get: x=33.6/4=8.35. So x=8.35.