Question

Two glasses together contain 280 mLof water. The first glass contains 80 ml more than the second glass. How much water does the first glass contain? 

Answer 1

X - 1st glass
y - 2nd glass

$x+y=280\\ x=y+80\\\\ y+80+y=280\\ 2y=200\\ y=100\hbox{ ml}\\\\ x=100+80\\ x=180\hbox{ ml}$