Farmer Bob owns a piece of grassland and three animals: a cow, a goat, and a goose. Bob discovered the following:

• When the cow and the goat graze on the field together, there is no more grass after 45 days.
• When the cow and the goose graze on the field together, there is no more grass after 60 days.
• When the cow grazes on the field alone, there is no more grass after 90 days.
• When the goat and the goose graze on the field together, there is no more grass after 90 days also.

The question: For how long can the three animals graze on the field together?

A hint : Click here!

The answer: Click here!