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?

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%