# subtraction of hours

• May 9th 2008, 10:05 AM
recca
subtraction of hours
Erin wants to figure out how much time elapsed between 9:45am and 11:30am. Erin does the following:
0 121
11:30
- 9:45
_______________
1:85
and says the answer is 1 hour and 85 minutes?

A Is Erin right? If not, explain what is wrong with her method, and show how to modify her method of regrouping to make it correct ( do not start from scratch

B Solve the problem of how much time elapsed between 9:45am and 11:30am, in another way than your modification of Erin's method. Explain your method.
• May 9th 2008, 10:12 AM
masters
How many of these problems do you have?

11:30 = 11 hrs 30 min
9:45 = 9 hrs 45 min

You can't subract 45 min from 30 min, so you borrow an hour from 11 and tack on the 60 min to the 30 min you already have:

10 hrs 90 min - 9 hrs 45 min = 1 hrs 45 min
• May 9th 2008, 10:40 AM
recca
I am studying for the teachers exam, and these are my last 20 problems that I need to know. Thanks for your help.
• May 9th 2008, 10:46 AM
r_maths
Quote:

Originally Posted by masters
10 hrs 90 min - 9 hrs 45 min = 2 hrs 45 min

1hr 45mins :)