To determine how many pints of lemonade Stephen needs to add to fill the bucket, we need to find the remaining volume of lemonade needed to reach the 5-gallon mark. Since 1 gallon is equal to 8 pints, we can calculate the remaining volume as follows:
5 gallons - 3.5 gallons = 1.5 gallons.
To convert this remaining volume to pints, we can multiply it by 8:
1.5 gallons x 8 pints/gallon = 12 pints.
Therefore, Stephen will need to add 12 pints of lemonade to fill the bucket.