Given that the family uses 312 gallons of milk.
Then, it is written as
1 week = 312 gallons
7 days = 312 gallons
D WE HAVE
1 day = 312/7 gallons
Now we need to find the milk required for 4 days. Then,
4 x 1 day = 4 x 312/7 gallons
4 days = 1248/7 gallons
4 days = 178.285 gallons
Therefore the family require 178.285 gallons for 4 days.
ON DIVIDING BY 7 BOTH SIDES WE HAVE
1 day = 312/7 gallons
as