digits

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lala
Posts: 18
Joined: Fri Mar 07, 2008 10:27 am

digits

Post by lala »

Hi
anyone know this answer, could you please explain this answer?

These are the four two-digit numbers that can be made with the digits 1and 7.

11 17 71 77

How many three-digit numbers can be made with the digits 1,2 and 3?.

(a) 6
(b) 9
(c) 21
(d) 27
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Post by Guest55 »

27 I think

each time there are three numbers to chose from so 3 x 3 x 3
KenR
Posts: 1506
Joined: Fri Mar 17, 2006 6:12 pm
Location: Birmingham

Post by KenR »

Hi Lala

27 is incorrect

The question is asking for the number of permutatons of 3 numbers taken three at a time.

There is a matherical formula which is: p = n!/ (n-r)!

Where n! means factorial = n x (n-1) x (n-2).....x 1

Hence for this p = 3 x2 x1 = 6

You can also work this ourt using the various sequences:-

Start with 1 in the Hundreds column: there are 2 options 123 and 132

With 2 you have 213 and 231

and with 3 you have 312 and 321

A total of 6.

Ken
fm

Post by fm »

Dear Ken,

If you read the question (as we're always telling our kids!) you seem to be able to use the digits more than once. So you can have the 27 I think.

e.g.

111, 112, 121, 122, 113, 131, 133, 123, 132,

222, 212, 211, 221, 213,231, 233, 223, 232,

333, 311, 312, 321, 322, 331, 332, 323, 313

Something like that.

Regards,

Fm
KenR
Posts: 1506
Joined: Fri Mar 17, 2006 6:12 pm
Location: Birmingham

Post by KenR »

Hi fm

You are quite correct - didn't spot you can reuse the digits
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