First of all, a whole day has 24 hours. So you need to compare
6 hours= 4 liters
24 hours= Y
Y represents the unknown
Cross and multiply. That will be
6 x Y=24 x 4
6Y=96
Divide both sides by 6
Y=96/6
Y=16
A whole day will be 16 liters. Hope I helped. Goodluck