Question

90% acid solution (90% pure acid and 10% water) and 97% acid solution are mixed to obtain 21 litres of 95% acid solution. How many litres of each solution are mixed.

Answer 1

Let the quantity of 90% acid solution be x litres and The quantity of 97% acid solution be y litres

According to the question,
x + y = 21                        ...(1)
and  90% of x + 97% of y = 95% of 21
90x + 97y = 1995            ...(2)

Multiplying equation no. (1) by 90, we get,
90x + 90y = 1890            ....(3)

Subtracting equation (2) from (3)
       90x + 90y = 1890
-      90x + 97y = 1995  
      -       -            -        
               - 7y = - 105
                   y = 15

From (1)
x + 15 = 21
x = 6
Hence, 90% acid solution is 6 litres and 97% acid solution is 15 litres.