3 1/2 pound = 7/2 pounds.
We know that the dog eats 7/2 (3.5) pounds every 21 days since there are 7 days in a week and Mr.Z only buys dog food every 3 weeks.
Set up a proportion. 3.5 lb = x
21 days 365 days
Cross multiply and divide. 3.5 lb x 365 days = 1277.5 / 21 days = 60.83 lb