Hello wiseowl,
I believe the answer is 25 gallons.
First I began by converting the 15 meters and 60 meters to feet and got
15 meters = 49.2126 feet and 60 meters = 196.85 feet.
Next I needed to find the area of the wall so I multiplied the numbers above and got 9687.50031 feet squared.
I rounded this up to 9688 feet squared.
Finally, I divided 9688 sqft by 400 sqft and got 24.22.
Since the problem is asking for the smallest amount of whole gallons, we round up 24.22 and get 25 gallons of paint.
Please let me know if I did anything wrong, it has been a while since I have done this math and may have done something wrong.
- Keihaku ;)