The correct option is C 72 gal
Jack sells 4 qt of juice on the first day. His sales increase by 1 pt every day.
So, on day 2: 1 pt and 4 qt of juice.
On day 3: 2 pt and 4 qt of juice.
On day 4: 3 pt and 4 qt of juice.
On day 5: 4 pt and 4 qt of juice.
Similarly, on day 21, he will sell 20 pt and 4 qt of juice.
We know,
2 pt = 1 qt
1 pt=12 qt
20 pt=202 qt=10 qt
Total amount of juice sold on day 21 = 4 + 10 = 14 qt
We also know, 4 qt = 1 gal
1 qt=14 gal
14 qt=144 gal
14 qt=72 gal
So, Jack will sell 72 gal of juice on day 21.