Question

how much 100% fire-fighting foam concentrate must be added to 1000 L of a 2.0% solution to make a 4.0% solution

Answer 1

100% FFFoam means its completely FFF.
Amount of FFF in 1000 Litres of 2.0% solution = 1000 * 2/100 = 20 litres

Concentration of FFF when X Litres added to 1000 Litrers of 2.0% solution :

   =>   (20 + X)  / (1000 + X)  = 4.0% = 4 / 100  = 1/25

 =>   25(20+X) = 1000 + X 

 =>   25 X + 500 = 1000 + X          X = 500/24 = 20.83 litres