A town is supplied with water from a big overhead tank which is fed with a constant volume of water regularly. When the tank is full, if 32000 gallons are used daily, the supply fails in 50 days. However, if 37000 gallons are used daily, the supply lasts for 40 days only. How much water can be used daily without the supply ever failing? `12000\ ga l lon s` b. `\ 18000\ ga l lon s` c. `15000\ ga l lon s` d. `20000\ ga l lon s`