# 4.2: Topic B- Time

- Page ID
- 95405

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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)### Topic B: Time

The ancient Babylonians used a number system based on 60. We still use their number system when we talk about time.

There are 60 minutes in an hour, and there are 60 seconds in a minute.

- 60 minutes = 1 hour
- 60 seconds = 1 minute

## Writing Time in Standard Format

Time is written in a standard format.

Hours: Minutes: Seconds

**Example:**

12 noon

would be written as 12:00:00

or 12:00 (without the seconds)

**Example: **

4 o’clock

would be written as 4:00:00

or 4:00 (without the seconds)

**Example:**

8 hours, 47 minutes, 3 seconds

would be written as 8:47:03

**Note:** When there is only one number, put in a zero to hold the tens place.

**Example:**

3 hours, 9 minutes, 3 seconds

would be written as 3:09:03

Exercise 1

Write the following times in standard format. Check your work using the answer key at the end of the exercise.

**Example:**

2 hours, 7 minutes, 31 seconds. 2:07:31

- 3 hours, 56 minutes, 42 seconds
- 12 hours, 2 minutes, 29 seconds
- 1 hour, 23 minutes, 54 seconds
- 6 hours, 7 minutes, 39 seconds
- 11 hours, 41 minutes
- 7 hours, 14 minutes, 59 seconds
- 21 hours, 36 minutes
- 1 hour, 51 minutes, 41 seconds
- 5 hours, 18 minutes, 10 seconds

**Answers to Exercise 1**

- 3:56:42
- 12:02:29
- 1:23:54
- 6:07:39
- 11:41
- 7:14:59
- 21:36
- 1:51:41
- 5:18:10

## A.M. and P.M.

You need to go to the dentist at 9:00 a.m. This is in the morning because of the **a.m. **The abbreviation **a.m. **means **ante meridiem **or **before noon. **We use a.m. for any times between 12 midnight and 12 noon.

You are meeting friends for dinner at 6:00 p.m. This is at night because of the **p.m. **The abbreviation **p.m. **means **post meridiem **or **after noon**. We use p.m. for any times between 12 noon and 12 midnight.

**Example:**

You catch the bus at 7 o’clock in the morning.

The time would be written as 7:00 a.m.

**Example:**

You are meeting friends to go fishing at 6:30 at night.

The time would be written as 6:30 p.m.

Exercise 2

Write the following times using a.m. or p.m. Check your work using the answer key at the end of the exercise.

**Example:**

The sun rises at 7:07 in the morning.

7:07 a.m.

- Your shift at work starts at 8:30 in the morning.
- Your class starts at 1:00 in the afternoon.
- Your son has soccer practice at 4:00 in the afternoon.
- You catch your bus at 6:15 in the morning.
- You need to go to the doctor at 3:20 in the afternoon.

**Answers to Exercise 2**

- 8:30 a.m.
- 1:00 p.m.
- 4:00 p.m.
- 6:15 a.m.
- 3:20 p.m.

## Rounding Time

When you round time, if the minutes are more than thirty, you round up to the next number of hours. If the minutes are less than thirty, you remain at the same number of hours.

**Example:**

If it took 45 minutes to drive to school, you would round that to one hour because 45 minutes is greater than 30 minutes.

**Example:**

If it took one hour and 15 minutes to get to school by bus, you would round that to one hour because 15 minutes is less than 30 minutes.

**Example:**

If it took 8 hours and 37 minutes to complete the painting job, you would round that to 9 hours because 37 minutes is greater than 30 minutes.

Exercise 3

Round the following times to the nearest hour. Check your work using the answer key at the end of the exercise.

**Example:**

The movie lasted 3 hours and 13 minutes.

3 hours.

- You needed 2 hours and 15 minutes for grocery shopping.
- It took 1 hour and 50 minutes to cook dinner.
- You drove for 9 hours and 23 minutes.
- Your baby slept for 1 hour and 47 minutes.
- You visited with friends for 3 hours and 11 minutes.
- It took 2 hours and 35 minutes to play the hockey game.
- You rode on the bus for 1 hour and 28 minutes.
- You walked to work in 38 minutes.
- How long does it take you to get to school?

**Answers to Exercise 3**

- 2 hours
- 2 hours
- 9 hours
- 2 hours
- 3 hours
- 3 hours
- 1 hour
- 1 hour
- Check with your instructor.