A water tank is currently three-quarters full. If 2500 gallons are drained, the tank will be one-third full. How many gallons of water can the tank hold when full?

We know from question that:

3/4 * volume - 1/3 * volume = 2500

Solving this equation for volume gives:

Volume * (3/4 - 1/3) = 2500

Volume * (9/12 - 4/12) = 2500

Volume * 5/12 = 2500

Volume = 2500 / (5/12)

Volume = 2500 * 12 / 5 = 500 * 12 = 6000 gallons.

If the tank is full, there will be 5,000 gallons. Therefore, the answer is 5,000.