Adding Time

Date: 04/26/99 at 17:09:26
From: Tracy Schneekloth
Subject: Spacecraft

A spacecraft is launched at 7:45 a.m. on Monday. It is in flight for 5 
days 3 hours and 18 minutes. What time does the spacecraft land?

I don't understand it at all!

Date: 04/26/99 at 19:18:13
From: Doctor Teeple
Subject: Re: Spacecraft

Hi Tracy,

Thanks for writing Dr. Math.

Let's start from the beginning and see if we can help you understand 
this question a bit more. I'll do a similar one all the way through to 
give you an example.

First, what is the question asking? Well, it says the spacecraft is 
in space for a certain amount of time and then lands. So we need to 
add all of the time that the spacecraft is in space. 

Let's suppose that the spacecraft was in space for 4 days, 3 hours, 
and 22 minutes. (Remember, we're just doing a similar question 
here). For me, the best way to add time is to do it in parts. That 
means we'll add the minutes, hours, and days separately.

So we start with 7:45 am on Monday. What if the spacecraft was in the 
air for 22 minutes total? What's 22 minutes after 7:45 am? We can 
write it this way:

     7:45 a.m.
   +   22 minutes
   ------

If we add them up with regular addition, we get:

     7:45 a.m.
   +   22 minutes
   ------
    7:67 a.m.

Uh oh, something's wrong here, since there is no 7:67 a.m. We need to 
take into account that there are only 60 minutes in an hour. So we 
carry the 60 minutes and make it into 1 hour. Do you see how this now 
becomes 8:07 a.m.?

Now we need to add on the hours. We get:

     8:07 a.m.
   + 3    hours
   ------
    11:07 a.m.

That one's straightforward. Normally when you add hours, you need to 
watch out for when the answer is bigger than 12. If it is, then either 
you've gone from a.m. to p.m. on the same day, or you've gone from 
p.m. to a.m. on the next day and you need to add another day.

Now we need to add on the days. Since we're adding 4 days, we just 
count 4 days after Monday, which is:

   Monday    Tuesday    Wednesday    Thursday    Friday
   =========> 1 day ===> 2 days ====> 3 days ==> 4 days

Friday. So the spacecraft lands at 11:07 a.m. on Friday. 

Your answer can be found in a similar way.

So if it becomes overwhelming to add time and days, separate them and 
do them in parts. If you need more help with time, here is a page from 
the Dr. Math archives:

  Adding and Subtracting Units of Time
  http://mathforum.org/dr.math/problems/shellie12.11.98.html   

Also, please feel free to write back if you have more questions or 
you want to check your answer to this one.

- Doctor Teeple, The Math Forum
  http://mathforum.org/dr.math/