I would like to calculate the expense associated with the time usage of
something, given a rate per hour, and times formatted as Text such as below:

1535 1700

1535 represents 3:35 pm, and 1700 represents 5:00 pm. I can calculate the
difference between the two, and get 3:25 for example, but not sure how to
express/format this to get the desired 3 & 25/60 hours times "x" rate.

Thanks in advance.