A and B are solutions of acid and water. The ratios of water and acid in A and B are 4 : 5 and 1 : 2, respectively. If x litres of A is mixed with y litres of B, then the ratio of water and acid in the mixture becomes 8 : 13. What is x : y ?

3 : 4

            Acid    :    Water   :    Total

A    :       4      :        5      :       9       )× 7    = 63

B    :       1      :        2      :       3       )× 21  = 63

New:      8      :       13     :       21     )× 3    = 63

Use alligation method on acid: