Answer:
112.5 mL
Step-by-step explanation:
Let x represent the amount of grape juice to add. Then the total amount of grape juice in the mix is ...
0.15(1800) +x = 0.20(1800+x)
270 +x = 360 + 0.2x . . . . . . eliminate parentheses
0.8x = 90 . . . . . . . . . . . . . . . subtract 270+0.2x
x = 90/0.8 = 112.5 . . . . . . . . divide by the coefficient of x
112 mL of grape juice should be added to make a mix that is 20% grape juice.
