The difference in age between Declan's elder son and younger son is 10 years. If the elder son is 40 years old now, how old will the younger son be 30 years from now? Let's be accurate as possible.
If the elder son is 40 years old now, the younger son is 40 - 10 = 30 years old. In 30 years from now, the younger son will be 30 + 30 = 60 years old.
The answer: 60.