Question 564653
Solution A
Amount = 500 liters 
Concentration =15% =0.15

Solution B
Amount = x 
Concentration =100% = 1

Resultant Solution
Amount =500+x 
Concentration =20%=0.2

[Amount Solution A * Concentration A] + [Amount Solution B * Concentration of B] = Amount of Resultant * Concentration of resultant

(500)(0.15)+(x)(1)=(500+x)(.02)
75+1x=100+0.2x
1x-0.2x=100-75
0.8x=25
0.8x/0.8=25/8
x=20

He should add 20 liters of pure acide to make a 20% acid solution?