Ian has 12.2 ounce bottle of ketchup. he uses 1/20 of the ketchup every time he has a burger. how?

After the first use the bottle contains 12.2 x 19/20 ounces.

After the second use the bottle contains 12.2 x (19^2)/(20^2) ounces.

After the third use the bottle contains 12.2 x (19^3)/(20^3) ounces.

After the fourth use the bottle contains 12.2 x (19^4)/(20^4) ounces.

Punch that into your calculator and there is your answer.