Answer: 15 Hours
Step-by-step explanation:
Money he earns at Library = 10 dollars per Hour
Money he earns at Store = 8 dollars per Hour
10+8=18 dollars per hour
280 divided by 18
=280
18
=15 as Quotient and 10 as Remainder
Final Answer: He has to work 15 hours to earn 280 Dollars.