Question

A thousand litre tank contains 50 litres of a 25% brine solution. if you have plenty of 75% brine solution, how many litres of 75% do you need to add to the tank to get a 30% brine solution?

Answer 1

Let the total brine solution be x
Thus
25% of x = x/4
and 
75% of x = 3/4 x

according to question

25% of  x = 50 lit
thus
x = 50 * 4
 x = 200 lit
total volume of tank = 1000 lit
ths clearly
75% of x = 1000 - 200
                = 800 lit

Now 25 % of brine solution = 200 lit
1 %                                       = 200/25
                                             = 8 Lit
thus
30 % =  8 * 30
          = 240Lit
Thus
240 lit is required