Dan is 5 years older than David. Bridget is 8 years younger than David. If the total of their ages is 57, how old is the eldest of them

The oldest person is 25 years old

Step-by-step explanation:

Represent David's age by d.

Then Dan's age is d + 5, and Bridget's is d - 8.

The sum of these three ages is 57:

d + (d + 5) + (d - 8) = 57

This simplifies to

3d - 3 = 57, or 3d = 60.  Then d must be 20.

David is 20, Dan is 25 (5 years older than David), and Bridget is 12 (8 years younger than David).

Check:  Do these three ages add up to 57?

20 + 25 + 12 = 57   YES\

The oldest person is 25 years old.  That's Dan's age.