Reducing the water bill

My family and I have created an idea to have a contest of who can use the least amount of water each day. We will have one week and whoever has the least amount of water used recieves a $20 Target giftcard. I have came up with a chart that limits me to how much water I can use each day: brushing teeth-5 gallons taking showers-15 gallons washing hands-10 gallons washing other things such as food-5 gallons