Alaboratory technician needs to make a 60-liter batch of a 40% acid solution. how can the laboratory technician combine a batch of an acid solution that is pure acid with another that is 20% to get the desired concentration?

We have solutions of 20% acid and 40% acid.
We need 60 liters of 40% acid.
"t" is liters of 20% and "f" is liters of 40%
We have to set up two equations:
A) t + f = 60
B) .20t + 1.0f = (60*.40)
Multiplying equation A) by -.2
A) -.2t  -.2f  = -12 then adding that to B)
B) .2t + f = (24)
.8f = 12
forty per cent volume = 15 liters
A) t + f = 60
Twenty per cent volume = 45 liters
Source:
http://www.1728.org/mixture.htm

What am i doing here. add?