An 18% brine is salt water containing 18% salt. Raoul mixed some 18% brine with some 35% brine to make 85 L of 19% brine. The number of litres of the 35% brine that he put into the mix is?

0.18B + 0.35*[85 - B] ==0.19 * 85, solve for B

B ==80 Liters of 18% brine

85 - 80 ==5 Liters of 35% brine.