# Missing number sequence

Hi I know how to work these out. Just wasn't sure why in this example I divide by 5 instead of 4

15. X. X. X. X. 55

55-15=40
Dived the 40 by 5 gives you 7. Which is the answer. But why do I decide by 5 when four missing number? Or by 4 when three missing number?
07:52 Thu 17th Nov 2011
 puternut Imagine you have a fence with the first post 15 metres away from you and the last 55 metres away. You have also 4 more posts to put in equally between them. The Xs You measure it out and find you put them in at 8 metre gaps. However you need 5 pieces of fencing to put in between the posts - hence the dividing by 5 08:07 Thu 17th Nov 2011 Go To Best Answer

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 is 40 divided by 5=7? 07:55 Thu 17th Nov 2011 actuually 40 divided by 5 is 8 not 7 07:58 Thu 17th Nov 2011 There are 5 steps to get from 15 to 55 so you divide the distance between (40) them by 5 And as others have said the answer to that is 8 So the sequence goes up in 8s 15, 23, 31, 39, 47, 55 08:04 Thu 17th Nov 2011 Imagine you have a fence with the first post 15 metres away from you and the last 55 metres away. You have also 4 more posts to put in equally between them. The Xs You measure it out and find you put them in at 8 metre gaps. However you need 5 pieces of fencing to put in between the posts - hence the dividing by 5 08:07 Thu 17th Nov 2011 Question Author Sorry typo, typing on phone on train. Thanks the fence analogy works for me cheers 08:37 Thu 17th Nov 2011 The fence post analogy is good. Another way is to thing of steps on a staircase Another way to look at it anything like this is to take a simpler example. Suppose the sequence is 5, X, 25 The difference between 5 and 25 is 20. Dividing by one (the number of missing values) would give the wrong answer. There are two steps needed to get from 5 and 25 so we divide by 2: so each step is 20/2=10. Add 10 to your starting number gives your answer of 15 08:48 Thu 17th Nov 2011

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