Acar moving at a constant velocity never changes speed? a true b false

That's true. 'Velocity' means speed and direction of motion.  If velocity is 'constant', then the speed never changes, and neither does the direction. Constant velocity means motion at a steady speed in a straight line.

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