Missing numbers on number line

BigCon

7, _, _, _, _, 77

I've moved to the States and this was one of the questions my 5th grader got for homework.

I can't figure it out (without using decimals).
I'm probably missing something stupid?

Cheers!

dafunk

Just keep adding 2 times 7 to the previous number.

gondorff

BigCon wrote: »

7, _, _, _, _, 77

I've moved to the States and this was one of the questions my 5th grader got for homework.

I can't figure it out (without using decimals).
I'm probably missing something stupid?

Cheers!

what dafunk said.

(77-7)/5 = 14

FoxT

It is an arithmetic series.

The first term is a
2nd term is a+r
3rd term is a+2r
nth term is a(n-1)r

etc.

So you can look at

7, _, _, _, _, 77

as, a, a+r, a+2r, a+3r, ....etc.

a=7 - so you just need to solve for r.

Michael Collins

Pretty stupid question when it doesn't tell you how the sequence is related. This is the kind of question to give someone if you want to make them hate maths!

An arithmetic series is no doubt what they were getting at, however.

LeixlipRed

Yeh, seriously annoying question. If you just ask it like that there are infinitely many answers.

Gurgle

Michael Collins wrote: »

Pretty stupid question when it doesn't tell you how the sequence is related.

Figuring out how the sequence is related is the whole point of the question.

Michael Collins

Gurgle wrote: »

Figuring out how the sequence is related is the whole point of the question.

I don't mean they should tell you that "You add 14 each time" which would just be giving you the answer, but they should at least tell you it's an arithmetic series, otherwise as LeixlipRed says there's an infinte number of possible answers. (A geometric series and a common ratio of [latex] ^5\sqrt(11) [/latex] is another solution, for example).

OK so it may be assumed from the context of the question that it's an arithmetic series I suppose, if it's in the "Arithmetic series" chapter or whatever. Still smacks of one of those annoying supposed "IQ test" type questions to me.

dafunk

Yes, agreed. It shouldn't say 'number line' when it is clearly not a number line but a series. Tell your kid to tell the teacher she sucks at Maths (or should I say 'Math').

MathsManiac

But we don't know the context in which the question was asked. If it's part of a whole series of activities and exercises about taking repeated equal jumps along a numberline, then perhaps the context was enough to make the task clear. (Fifth graders in the US are about 10 or 11 years old.)

Gurgle

MathsManiac wrote: »

Fifth graders in the US are about 10 or 11 years old.

Until this very moment, I have lived (happily) without having a clue what 5th grade meant.

So its the same as 5th class?

MathsManiac

Gurgle wrote: »

Until this very moment, I have lived (happily) without having a clue what 5th grade meant.

So its the same as 5th class?

I'm not sure. My assertion was based on Wikipedia: http://en.wikipedia.org/wiki/Fifth_grade

However, that Wikipedia article says that the Irish equivalent is third class, which is is for 8-9 year olds. That doesn't seem right. It seems to be based on the fact that third class is the fifth year of primary school. However, in the US, and many other countries, there's also "Kindergarten" for a year or two, which I think you could regard as equivalent to infants. So I think saying that fifth class is about the same as fifth grade isn't too far off the mark. In the US, kindergarten is just 1 year, I think, so maybe there's a case to be made that 5th grade is equivalent to 4th class.

Mellor

MathsManiac wrote: »

In the US, kindergarten is just 1 year, I think, so maybe there's a case to be made that 5th grade is equivalent to 4th class.

It's the 6th year over all, so its the same at 4th class (assuming two years at infants)

Michael Collins wrote: »

I don't mean they should tell you that "You add 14 each time" which would just be giving you the answer, but they should at least tell you it's an arithmetic series, otherwise as LeixlipRed says there's an infinte number of possible answers. (A geometric series and a common ratio of [latex] ^5\sqrt(11) [/latex] is another solution, for example).

OK so it may be assumed from the context of the question that it's an arithmetic series I suppose, if it's in the "Arithmetic series" chapter or whatever. Still smacks of one of those annoying supposed "IQ test" type questions to me.

So, considering its for 9 year olds, the over is totally overboard ffs. They were probably working on similar stuff all week.
The answer is obviously the simplest one, were all terms are positive, addition, equal numbers.

RoundTower

Starting with n = 2, for each n, add the next n numbers.

n = 2: 3 + 4 = 7
n = 3: 4 + 5 + 6 = 15
n = 4: 5 + 6 + 7 + 8 = 26
n = 5: 6 + 7 + 8 + 9 + 10 = 40
n = 6: 7 + 8 + 9 + 10 + 11 + 12 = 57
n = 7: 8 + 9 + 10 + 11 + 12 + 13 + 14 = 77

I'm sure this sequence comes up all the time in whatever branch of analytical number theory the child is studying. Don't knock the American educational system!

red_fox

There are many good options:

http://oeis.org/search?q=7, _,_,_,_,77

RoundTower may have the nicest.

ray giraffe

RoundTower wrote: »

Don't knock the American educational system!

If we're talking about American education, it must be a bible reference.

Numbers 7:77

"and two oxen, five rams, five male goats and five male lambs a year old, to be sacrificed as a fellowship offering. This was the offering of Pagiel son of Ocran."

So 7:77 contains the 4 sacrificial numbers 2,5,5,5 - the secret code must be 7,2,5,5,5,77.

We're through the looking glass here people. :eek: