solve for x 9 = X + (10-X) * .8
9 = x + (10 - x) * .8
Use the distributive property:
9 = x + 8 - 0.8x
Subtract 8 from both sides:
1 = x - 0.8x
Combine like terms:
1 = 0.2x
Divide both sides by 0.2:
5 = x