How do you do this?

a pineapple drink contains 15% pineapple juice. how much pure pineapple juice should be added to 8 quarts of the drink to obtain a mixture containing 50% pineapple juice?

Let X be the amount of pure pineapple that you must add, in quarts. The amount of pure pineapple in the mix will be
(0.15)*8 + X
This must equal the amount of water (or non-pineapple ingredients), which is 0.85*8.
1.2 X = 6.8

Solve for X

Caleb has 1 liter of a mixture containing 42% boric acid. How much water must be added to make the mixture 6% boric acid?

Let's denote the amount of water that needs to be added in liters as X.

The quantity of boric acid in the original mixture is given by (0.42 * 1) liters, and the quantity of boric acid in the final mixture is (0.06 * (1 + X)) liters.

Since we are adding water, the total volume of the final mixture will be (1 + X) liters.

We can set up the equation:
0.42 * 1 = 0.06 * (1 + X)

Solving for X:

0.42 = 0.06 + 0.06X
0.42 - 0.06 = 0.06X
0.36 = 0.06X

Dividing both sides of the equation by 0.06:
X = 6

Therefore, 6 liters of water must be added to make the mixture 6% boric acid.