"What is the approximate magnitude of the electric field between the charge center and the ground?What is the approximate magnitude of the electric field between the charge center and the ground?"

the current is 0.6

voltage ÷ resistance = current

so 12 ÷ 20 = 0.6

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