Title: Lesson 12-6
Author: mark_cameron
Last saved by: MHE
Company: McGraw-Hill

Lesson 8-7

Example 1 Add Units of Time
Find the sum of 3 h 12 min and 1 h 35 min.

Estimate 3 h 12 min + 1 h 35 min ≈ 3 h + 1.5 h or 4.5 h

    3 h 12 min	Add minutes first, then hours.
+  1 h 35 min    
    4 h 47 min

Check for Reasonableness 4 h 47 min ≈ 4.5 h ✓

Example 2 Subtract Units of Time
Find the difference of 6 h 32 min 44 s and 4 h 53 min 40 s.

Estimate 6 h 32 min 44s − 4 h 53 min 40 s ≈ 6.5 h − 5 h or 1.5 h
 
    6 h 32 min 44 s	Subtract the seconds first. Notice that you cannot subtract 53 
−  4 h 53 min 40 s 	minutes from 32 minutes.
      	4 s

    (5 h 60 min) 32 min 44 s	Rename 6 h as 5 h + 1 h or 5 h 60 min. 	 
−   4 h               53 min 40 s 	
	4 s

    5 h 92 min 44 s 	Add the minutes.
−  4 h 53 min 40 s
	4 s

    5 h 92 min 44 s 	Subtract the minutes, then the hours.
−  4 h 53 min 40 s	 
    1 h 39 min   4 s

Check for Reasonableness 1 h 39 min 4 s ≈ 1.5 h ✓

Example 3 Real-World Example
TYPING   Jessica took 3 h 45 min 35 s to type her report and Colin took 
3 h 18 min 56 s to type his report. How much longer did it take Jessica?

Estimate	Since the numbers are so close, use the minutes.
		3 h 45 min 35 s − 3 h 18 min 56 s ≈ 45 min − 20 min or 25 min

    3 h 45 min 35 s	Since you cannot subtract 56 seconds from 35 seconds, you must 
−  3 h 18 min 56 s 	rename 45 min 35 seconds as 44 minutes 95 seconds.
      
    3 h 44 min 95 s 	Subtract seconds first, then minutes, and finally the hours.
−  3 h 18 min 56 s 
          26 min 39 s

It took Jessica 26 minutes 39 seconds longer to type her report. 

Check for Reasonableness 26 min 39 s ≈ 25 min ✓

Example 4 Elapsed Time
TRAVEL   A flight leaves Pittsburgh at 10:45 A.M. and arrives in Atlanta at 
3:05 P.M. How long is the flight?

You need to find how much time has elapsed.

	10:45 A.M. to 12:00 noon	12:00 noon to 3:05 P.M.
	is 1 hour 15 minutes.	is 3 hours 5 minutes.

Add the elapsed time before noon and the elapsed time after noon to find the total elapsed time.

   1 h 15 min
+ 3 h   5 min
   4 h 20 min

The length of the flight is 4 hours 20 minutes.