Alright, lets get started.
Suppose 1 barrel of the grass contains amount of grass = x
The bisons at the city zoo are fed
of a barrel of grass each day.
Means bisons are fed amount of grass =

The elephants are fed
as much grass as the bisons.
Means amount of grass elephants are fed per day =

Means amount of grass elephants are fed per day =

of a week means =
days
Means amount of grass elephants are fed in 4 days =

It means
barrels of grass are fed to elephants in 4 days. : Answer
Hope it will help :)