before dancing on a smooth wooden floor, ballet dancers sometimes sometimes put a sticky powered called rosin on their shoes soles. Why? what force are they taking advantage of?

Okay, so they want to basically Increase their grip, and they are taking advantage of the force of friction

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