1 unit of solution X contains 0.15 units of salt: 1 unit of solution Y contains 0.08 units of salt. So A units of solution X plus 5 units of solution Y contain 0.15A + 0.40 units of salt in A+5 units. If this is 10% salt then 0.15A + 0.40 = 0.10(A+5), that is, 0.15A + 0.40 = 0.10A + 0.50. Simplifying, 0.05A = 0.10, so that A = 2, as stated.