Okay, first before we make an equation let change everything in to sixths. 2/3=4/6 and 1 1/2=9/6... Just so you know X = the amount of milk we want... So you equation is: 4/6 + X = 9/6
so, first we switch it around so that the X is on the left. Because this is addition we can do that without a problem.
X + 4/6 = 9/6 now we subtract 4/6 from both sides. X + 4/6 - 4/6 = 9/6 - 4/6 and since
9/6 - 4/6 = 5/6 you need 5/6 more cups of milk
