Which of the following does not use an internal combustion enginea. a cannonb. a lawnmowerc. a windmilld. a car

c) a Windmill

Explanation:

A lawn mower may have a two stroke or a 4 stroke internal combustion engine.

Wind mill is a device that runs with wind. When wind blows, wings of the wind mill rotate. Wind mills are used to generate electricity. It is a renewable source of energy that requires large farms or open space for their establishment and maintenance.

In a cannon gun powder is ignited ,  not with the help of an engine , but a different method for its ignition.

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