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%

THE TRICK IS TO JUST USE ONLY ONE VARIABLE

Find the equation for the area of a circle and divide that by 2