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