0.5cm The water shoots 12.0m and is acted upon only by gravity in the air. We can find the velocity it left the nozzle with from projectile motion. First we find the time it takes for the water to fall 1.0m from rest. Then we plug in the time along with the distance the water goes in the x-direction to find the velocity.
The speed of the water in the nozzle must be 26.6m/s. We use this with the conservation of flow rate to find the velocity of the water in the tank . To find the pressure in the tank, we use the equation relating the pressure in the tank with the pressure in the nozzle . We then plug in numbers and solve for the Pressure in the tank.
The atmosphere makes 
of that pressure possible, so only 
is created by the human.
