If Ian uses 1/20 of the ketchup 4 times, this is equal to 4/20, or 

.

 can be simplified to 

, which means we need to work out 

 of 12.2

 is equivalent to 20%, which as a decimal is 0.2
0.2 x 12.2 = 2.44 - however, this is how much he has 
used, and we need to figure out how much is left.
12.2 - 2.44 = 9.76
So, Ian has 9.76 oz remaining :)