Math Is Fun Forum

Toast

Real Member

Registered: 2006-10-08

Posts: 1,321

Formula Challenge

Alright, let's see if you can come up with a general, all encompassing formula for the question:

How many (No. between 0 and 9 inclusive)'s are there between x and y?

I tried to make one at school during Christian Ed class but failed miserably, is it actually possible? Have a go...

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Formula Challenge

I'm not sure what the question is. Could you explain it a bit more?
For example, let's say that x = 10 and y = 20. With these values, what would the thing that the formula is supposed to find be?

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It wanted to be normal.

Ricky

Moderator

Registered: 2005-12-04

Posts: 3,791

Re: Formula Challenge

10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 which is all 10 digits.

On the other hand, between [4, 6] there are three, and between [19,21] there are {19, 20, 21} 4.

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"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

Zach

Member

Registered: 2005-03-23

Posts: 2,075

Re: Formula Challenge

It seems to be the simpe idea that the number of integers ending in 0-9 inclusive is Y - X... If I'm understanding correctly.

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Boy let me tell you what:
I bet you didn't know it, but I'm a fiddle player too.
And if you'd care to take a dare, I'll make a bet with you.

Toast

Real Member

Registered: 2006-10-08

Posts: 1,321

Re: Formula Challenge

Woops, I meant, for instance:

How many 7s are there between 1 and 100, and the answer would be 20 ( i think)

MathsIsFun

Administrator

Registered: 2005-01-21

Posts: 7,713

Re: Formula Challenge

Is "77" two 7s?

(Geez that sounds a stupid question!)

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"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

Toast

Real Member

Registered: 2006-10-08

Posts: 1,321

Re: Formula Challenge

Yep