If 1800ml of a juice drink is 15% grape juice, how much grape juice should be added to make a drink that is 20% grape juice?

  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.

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