Question

You and your friend are making 30 liters of sodium water. You have liters of 10% sodium and your friend has liters of 22% sodium. How many of your liters and how many of your friends liters should you mix to make 30 liters of 15% sodium?

Answer 1

x liters of 10% + (30 - x) liters of 22% = 30 liters of 15%

Plugging in the numbers
x(10) + (30 - x)22 = 30(15)

Distribute
10x + 660 - 22x = 450

Combine like terms
-12x = -210

Divide both sides by -12
x = 17.5
30 - x = 12.5

17.5 liters of 10% + 12.5 liters of 22% = 30 liters of 15%

THE TRICK IS TO JUST USE ONLY ONE VARIABLE