Rounding – What’s my number?

I used a little activity with my year 8 that i thought of whilst teaching them. They were looking at Bounds and we were discussing which numbers equal 5 when rounded to the nearest whole number. They could list 4.5, 4.6, 4.7, 4.8, 4.9, 5.1, 5.2, 5.3, 5.4 but didn’t pick up on 5 itself or all the other numbers from 4.5 and up to but not including 5.5. So i told them i was thinking of a number they could ask me a question like; ‘What is your number rounded to the nearest 10’ or something similar and when they thought they had my number they could guess it.

My first number was 512.

They asked ‘What is your number rounded to the nearest 100?’. I said 500

Next they asked; ‘What is your number rounded to the nearest 10?. I said 510

Next they asked; ‘What is your number rounded to the nearest whole number?. I said 512.

Then a student guessed 512. 

Next i chose the number 430.6991

Feeling a little smarter they asked ‘What is your number rounded to the nearest 1 decimal place?’. I said 430.7

Next they asked; ‘What is your number rounded to the nearest 2 decimal places?. I said 430.70

Then a student guessed 430.7

I said no. They were very confused at this point. After a while a student said  ‘What is your number rounded to the nearest 3 decimal places?. I said 430.699

Then a student guessed 430.699

My next number was going to be 400.0005 but we ran out of time!

 

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