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Odds, Evens and More Evens

Age 11 to 14
Challenge Level Yellow star
Secondary curriculum
  • Problem
  • Getting Started
  • Student Solutions
  • Teachers' Resources

Odds, Evens and More Evens printable worksheet
 

Here are the first few sequences from a family of related sequences:
 
$A_0 = 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29...$
 
$A_1 = 2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42...$
 
$A_2 = 4, 12, 20, 28, 36, 44, 52, 60...$
 
$A_3 = 8, 24, 40, 56, 72, 88, 104...$
 
$A_4 = 16, 48, 80, 112, 144...$
 
$A_5 = 32, 96, 160...$
 
$A_6 = 64...$
 
$A_7 = ...$
 
.
.
.

Which sequences will contain the number 1000?
 
Once you've had a chance to think about it, click below to see how three different students began working on the task.
 
Alison started by thinking:

"I have noticed that each number is double the number in the row above. I wonder if I can work out what would go in the rows above 1000?"


Bernard started by thinking:

"I have noticed that in $A_1$, the numbers which end in a 0 are 10, 30, 50... If I carry on going up in 20s I won't hit 1000, so I know 1000 isn't in $A_1$."

 
Charlie started by thinking:

"I have noticed that each number in $A_1$ is 2 more than a multiple of 4. I know 1000 is $250 \times 4$ so it can't be in $A_1$."

 

Can you take each of their starting ideas and develop them into a solution?

Here are some further questions you might like to consider:
 
How many of the numbers from 1 to 63 appear in the first sequence? The second sequence? ...  
 
Do all positive whole numbers appear in a sequence?
Do any numbers appear more than once?
Which sequence will be the longest?  
 
Given any number, how can you work out in which sequence it belongs?
How can you describe the $n^{th}$ term in the sequence $A_0$? $A_1$? $A_2$? ... $A_m$?

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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