Charlie buys a 40 pound bag of cat food. His cat eats a 1/2 pound of food per day.

[math_celebrity]

Charlie buys a 40 pound bag of cat food. His cat eats a 1/2 pound of food per day.

Set up an equation:
1/2x = 40 where x is the number of days

Multiply through by 2
x = 80