There are 4 * 7 = 28 days in those 4 weeks. That means he takes 28 / 2 = 14 showers. So he takes a total of 14 * 10 = 140 minutes of showers. So he uses 2 * 140 = 280 gallons of water.
This justifies what answer for what question? Q & A:
John takes a 10-minute shower every other day for 4 weeks. If his shower uses 2 gallons of water per minute. How much water does he use in those 4 weeks?
280