if he uses 3 1/6 gallons for 1/3 sqaure foot of garden, then he must use
3 1/6 * 3 gallons for every 1/3 * 3(1 sqaure foot) sqaure foot of garden.
3 1/6 = 19/6 gallons,
19/6 * 3 = 19/2 gallons, 1/3 * 3 = 3/3 = 1 square foot
So, he uses 19/2 (also 6 1/2) gallons of water per sqaure foot to water his garden every week.