Subject :IMO Class : Class 3

How many four digit numbers can be formed, using 7, 5, 0, 2 only once in a number?

A 4

B 12

C 9

D 18

Do we have formulae to solve this problem?

## Ans 1:

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## Ans 2:

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## Ans 4:

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## Ans 6:

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Vibhuraman
from
NPS, Bangalore

Class : Class 3

n!-(n-1)! is the answer. n is number of digits here. (n-1)! is subtracted from n! since there is digit with 0. otherwise it is just n!
Now How do you calculate n!. For example 4! here.
4! = 4x3x2x1 = 24
(4-1)! = 3! = 3x2x1 = 6
Hence the answer is n! - (n-1)! = 4! - (4-1)! = 4! - 3! = 24 -6 = 18
Option D is the answer.

## Ans 9:

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JIYA GOSWAMI
from
DPS, Ranchi

Class : Class 6

Using Factorial for Class 3 seems difficult.
I taught my daughter as under:-
Step 1 - Put 7 at thousand place and how many different numbers we can get --She replied 6
Step 2- Put 5 at thousand place and how many different numbers we can get --She replied 6
Step 3 -Put 0 at thousand place , do we get four digit number --She replied NO
Ste 4 - Put 2 at thousand place and how many different numbers we can get --She replied 6
Step 5 - Add numbers obtained in above steps we get 18