It would take 45 days for all of the water to run out.
Step-by-step explanation:
Every 3 days, the barrel leaks 5 gallons. After the first three days, it would be 55 gallons. After 6 days, it would have been 50 gallons, but 15 gallons of water was added (bringing it up to 65 gallons). Going in increments of 3 (for 3 days), count backwards by 5. (Ex. 9 = 60, 12 = 55, 15 = 50, etc.) The barrel is completely empty after 45 dayd.