Five years ago, the sum of Sebastian's age and his sister's age was 3/4 of their father's age. How old is their father today if Sebastian is 40 years old and 10 years older than his sister?
A step-by-step solution is:
85
If Sebastian is 40 years old and 10 years older than his sister, his sister is 40 - 10 = 30 years old.

Five years ago, Sebastian was 40 - 5 = 35 years old.

Similarly, Sebastian's sister's age was 30 - 5 = 25 years old.

The sum of their ages five years ago was 25 + 35 = 60.

Five years ago, the sum of Sebastian's age and his sister's age was 3 / 4 of their father's age, meaning their father's age was 60 / 3 / 4 = 80 years.

The father's age is now 80 + 5 = 85.

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So, the final answer is 85