Current can flow only downhill in a water conduit because of:

Gravity

Explanation:

Gravitational force is acting on every object on this Earth and the direction is downwards. The same force will be acting naturally on the water current as well. As the force will be acting downwards the water current will always be downhill. If you need the water to flow uphill you will need to apply external force on water to make it move upward.

in physics zero. in muscle fatigue could be high

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form