Kim started to nap at 115 and woke up at 144.
Let's find the number of minutes that she napped.
I believe this is 1:15 not 115 because there's no such time.
Same goes with 144, its 1:44 and not 144.
Now, to be able to get the number of minutes that Kim napped,
Simply subtract the time she woke up by the time she started to take a nap
.=> 1:44 - 1: 15
=> 29 minutes is the number of minutes that Kim napped.