# Think of a Number

## Ten students think of a number then perform various operations on that number. You have to find what the original numbers were.

##### Level 1Level 2Level 3DescriptionHelpAlgebra

This is level 3: Negative decimal THOAN problems. You can earn a trophy if you get at least 7 questions correct.

 $$\times 2$$$$+25$$$$13$$Ethan says:'I think of a number. I double my number and add 25 the answer is 13'What number was Ethan first thinking of? Working: Andrew says:'I am also thinking of a number. If I multiply my number by 2 and add 6 the answer is 3.6'What number was Andrew first thinking of? Working: Daniel says:'I think of a number. I multiply my number by 3 and add 8.2 the answer is -11.3'What number was Daniel first thinking of? Working: Christopher says:'Can you guess what number I am thinking of if when I multiply it by 6.3 and add 2.2 the answer is -26.78?'What number was Christopher first thinking of? Working: Joseph says:'If I divide my number by 5 and add 9.2 the answer is 7.6'What number was Joseph first thinking of? Working: William says:'If I multiply my number by 5.6 then add 1.3 then add 6 then add 4.6 the answer is -31.78'What number was William first thinking of? Working: David says:'I am thinking of my favourite number. If I add 10.2 to my number then divide the result by 2, the answer is 2.25'What number was David first thinking of? Working: Nicholas says:'My plan was to add 7.3 to my number then multiply the result by 2. Then I multiplied the result by 5.1 and finally subtracted 9.5. The answer is 42.52'What number was Nicholas first thinking of? Working: James says:'I decided to add 6.2 to my number then multiply the result by 3, After that I subtracted 8.2 then doubled the result. The answer is -0.199999999999996'What number was James first thinking of? Working: John says:'If I multiply my number by 9.4 and subtract 7.7 then multiply the result by 6.4, the answer is 9.3 less than -268.588'What number was John first thinking of? Working:
Check

This is Think of a Number level 3. You can also try:
Level 1 Level 2

## Instructions

Try your best to answer the questions above. Type your answers into the boxes provided leaving no spaces. As you work through the exercise regularly click the "check" button. If you have any wrong answers, do your best to do corrections but if there is anything you don't understand, please ask your teacher for help.

When you have got all of the questions correct you may want to print out this page and paste it into your exercise book. If you keep your work in an ePortfolio you could take a screen shot of your answers and paste that into your Maths file.

## Transum.org

This web site contains over a thousand free mathematical activities for teachers and pupils. Click here to go to the main page which links to all of the resources available.

## More Activities:

Mathematicians are not the people who find Maths easy; they are the people who enjoy how mystifying, puzzling and hard it is. Are you a mathematician?

Comment recorded on the 19 June 'Starter of the Day' page by Nikki Jordan, Braunton School, Devon:

"Excellent. Thank you very much for a fabulous set of starters. I use the 'weekenders' if the daily ones are not quite what I want. Brilliant and much appreciated."

Comment recorded on the 28 May 'Starter of the Day' page by L Smith, Colwyn Bay:

"An absolutely brilliant resource. Only recently been discovered but is used daily with all my classes. It is particularly useful when things can be saved for further use. Thank you!"

Each month a newsletter is published containing details of the new additions to the Transum website and a new puzzle of the month.

The newsletter is then duplicated as a podcast which is available on the major delivery networks. You can listen to the podcast while you are commuting, exercising or relaxing.

Transum breaking news is available on Twitter @Transum and if that's not enough there is also a Transum Facebook page.

#### Three Ways

Can you get to the target number by multiplying together four different one digit numbers? Can you do it in three different ways? There are nine levels to this online challenge and a virtual Transum Trophy available for each level.

There are answers to this exercise but they are available in this space to teachers, tutors and parents who have logged in to their Transum subscription on this computer.

A Transum subscription unlocks the answers to the online exercises, quizzes and puzzles. It also provides the teacher with access to quality external links on each of the Transum Topic pages and the facility to add to the collection themselves.

Subscribers can manage class lists, lesson plans and assessment data in the Class Admin application and have access to reports of the Transum Trophies earned by class members.

Subscribe

## Go Maths

Learning and understanding Mathematics, at every level, requires learner engagement. Mathematics is not a spectator sport. Sometimes traditional teaching fails to actively involve students. One way to address the problem is through the use of interactive activities and this web site provides many of those. The Go Maths page is an alphabetical list of free activities designed for students in Secondary/High school.

## Maths Map

Are you looking for something specific? An exercise to supplement the topic you are studying at school at the moment perhaps. Navigate using our Maths Map to find exercises, puzzles and Maths lesson starters grouped by topic.

## Teachers

If you found this activity useful don't forget to record it in your scheme of work or learning management system. The short URL, ready to be copied and pasted, is as follows:

Alternatively, if you use Google Classroom, all you have to do is click on the green icon below in order to add this activity to one of your classes.

It may be worth remembering that if Transum.org should go offline for whatever reason, there are mirror sites at Transum.com and Transum.info that contain most of the resources that are available here on Transum.org.

When planning to use technology in your lesson always have a plan B!

Do you have any comments? It is always useful to receive feedback and helps make this free resource even more useful for those learning Mathematics anywhere in the world. Click here to enter your comments.

For Students:

For All:

## Description of Levels

Close

Level 1 - Whole number THOAN problems

Level 2 - Decimal THOAN problems

Level 3 - Negative decimal THOAN problems

Answers to this exercise are available lower down this page when you are logged in to your Transum account. If you don’t yet have a Transum subscription one can be very quickly set up if you are a teacher, tutor or parent.

## Example

Daniel says 'If I double my number and add 6 the answer is 30'

### Method 1 - Work backwards

Start at the end of the sentence with the answer 30 and undo all of the operations Daniel performed on his number in reverse order.

30 minus 6 then divide by two

The number Daniel was thinking about was 12

### Method 2 - Form and solve an equation

Let the number that Daniel is first thinking of be n.

2n + 6 = 30

Subtract 6 from both sides

2n = 24

Divide both sides by 2

n = 12

The number Daniel was thinking about was 12

Don't wait until you have finished the exercise before you click on the 'Check' button. Click it often as you work through the questions to see if you are answering them correctly. You can double-click the 'Check' button to make it float at the bottom of your screen.

Answers to this exercise are available lower down this page when you are logged in to your Transum account. If you don’t yet have a Transum subscription one can be very quickly set up if you are a teacher, tutor or parent.

Close