Let A, B represent the amount of 20% drink and 10% drink.
A+B = 10
0.2A + 0.1B = 0.13*10
Use substitution
From 1st equation
A = 10 - B
Substituting into 2nd equation
0.2(10-B) + 0.1B = 1.3
Distribute
2 - 0.2B +0.1B = 1.3
Combine like terms
-0.1B = -0.7
Divide by -.1 on both sides
B = 7
Substituting back into 1st equation
A = 10 - 7 = 3
Therefore 3L of 20% drink should be mixed with 7L of 10% drink.
