If a pound of guinea-pig pellets is about 3 cups of food, then there are 3/1 = 3 cups of pellets per pound. If each guinea pig eats 1/4 cup of pellets per day, then one pound of pellets will last 3/(1/4) = 12 days for one guinea pig.
To see why, we can use the following formula:
Number of days = Total amount of pellets / (Amount of pellets eaten per day x Number of guinea pigs)
In this case, we have:
Number of days = 3 / (1/4) = 12
This means that one pound of pellets will last 12 days for one guinea pig. If there are more than one guinea pig, we need to adjust the formula accordingly.