Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Calculating Timecards


Date: 08/03/99 at 13:01:40
From: Sandy Felker
Subject: Calculating time

Hi. I hope you can help me. I calculate timecards for hours and I'm 
wondering if there is a shortcut for calculating time. For example, 
say a person arrives at 6:08 a.m. and leaves at 3:49 p.m. Is there a 
quick formula to determine how many hours that person worked?


Date: 08/03/99 at 22:02:55
From: Doctor Peterson
Subject: Re: Calculating time

Hi, Sandy.

I have an easy method and a shortcut. You can decide which is better.

Here's the standard method: 

First, deal with the AM and PM by adding 12 to the latter, converting 
them both to 24 hour time. (If the times were PM and AM, you could 
still do the same thing, adding 12 to the later time to account for 
the passing of midnight.) 

Now you want to subtract 6:08 from 15:49.

       15:49
     -  6:08
     -------
        9:41

That was too easy, because we didn't have to do any borrowing! Let's 
do another, subtracting 6:49 a.m. from 3:12 p.m. (15:12).

       15:12
     -  6:49
     -------

We can't subtract 49 from 12, so what do we do? We borrow from the 
hours, remembering that 1 hour is 60 minutes; we subtract one from the 
hours and add 60 to the minutes:

       14:72  <-- was 15 hours and 12 minutes
     -  6:49
     -------
        8:23

Now here's the shortcut, which you may find easier or harder. Break 
the time interval you want to measure into pieces, like this:

6:49 a.m. ---> 7:00 a.m. ---> 12:00 noon ---> 3:00 p.m. ---> 3:12 p.m.
          60-49           12-7
         =11 min         =5 hr            3 hr          12 min
            |              |               |              |
            |              +---------------+              |
            |                 5+3 = 8 hr                  |
            |                                             |
            +---------------------------------------------+
                             11+12 = 23 min

The total time elapsed is 8:23, as before. If you find this easier to 
do in your head, use it; if not, use the other method.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/   
    
Associated Topics:
Elementary Calendars/Dates/Time
Elementary Subtraction
Middle School Calendars/Dates/Time

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/