One way to find out how long someone spends doing something is to subtract the time they finish from the time they start. Here's an example:
I start practicing my guitar at 5:13 and I
keep practicing until 5:37. How long do I
spend practicing my guitar?
ANSWER:
5:37
- 5:13
--------
:24
Jill starts washing the dishes at 11:21 and she
finishes washing the dishes at 11:30. How long
did it take Jill to wash the dishes?
ANSWER:
11:30
- 11:21
--------
:09