What is the value of x in the equation 4x + 8y=40 when y=0.8

8*.8=6.4 
4x+6.4=40
40-6.4=33.6
33.6/4=x
8.4=x

Hope this helps :)

To solve this equation, first you must plug in the given y value.
This leaves you with:
4x + 8(0.8) = 40

Follow PEMDAS (parentheses, exponents, multiplication, division, addition, subtraction) to simplify the parenthesis.
After multiplying, you get:
4x + 6.4 = 40

Continue following PEMDAS to subtract 6.4 from both sides of the equal sign. Make sure to subtract on both sides so the equation remains even and balanced.
This leaves you with:
4x = 33.6

From here, isolate your variable, x, by dividing by four on both sides.
Leaving you with:
x = 8.4

So your final answer is x = 8.4 when y = 0.8