Question

how many four digit number in all they are in 10 system​

Answer 1

Let's do it in a different way.

say we have a four digit number abcd

a_b_c_d .

now in our number system we have number from

0 to 9 and we need to replace abc and d from by

these number.

let's first replace a . as number can start with zero

so we can replace a in 9 way i.e by 123456789

when it comes to replace b, we can replace it in ten ways i.e 0123456789

In similar way we can replace c and c in ten ten

ways .

so we get

9_10_10_10

thus the total four digit number in decimal system

is = 9×10×10×10=9000

And their simple way to answer this is given below.

Smallest four digit number = 1000

Largest four digit number = 9999

so, total four digit number

( 9999- 1000) + 1

9000

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