To calculate this, we have to establish a proportion :
If 1 gallon of milk is drunk in 3 days,
X gallons will be drunk in 5 days
X gallons = 1 gallon*5days/3days = 1.66 gallons will drink in 5 days.
We can round to the first decimal place 1.66 ≈ 1.7 gallons approximately.
Answer : 1.7 gallons will Brian family drink in five days, approximately.
Hope this helps!