Possible capacity of Blue punch jug = 21 gallons
In the question,
Total number of jugs = 6
Capacities of jugs = 13, 15, 16, 21, 21, 22
Total capacity of all the jugs = 108 gallons
One of the jug is filled with Blue Punch.
Other remaining Five Jugs, with Red punch.
As,
He distributed the Red Punch in 2 parts of 1/3 and 2/3.
1/3 to Randy and 2/3 to Lisa.
So,
There are only two possibilities for which he can have the Blue jug Punch all to himself of the capacity,
Possibility 1 :
Blue Punch = 15 gallons
Red Punch = 108 - 15 = 93 gallons
Now, one third will be 31 gallons which is not a possible portion without opening the jugs.
Therefore, the 15 gallons of Blue Punch can not be a case.
Possibility 2 :
Blue Punch = 21 gallons
Red Punch = 108 - 21 = 87 gallons
One-third will be = 29 gallons which can be a possible combination of 13 and 16 gallons jug.
Therefore, the Blue punch Jug will be of 21 gallons capacity.