Question

How many five digits numbers are there in all​

Answer 1

The smallest 5 digit number is 10000. The largest is 99999. 99999 minus 10000 is 89999, but since you have a zero-based count, you have to add 1. That means there are a total of 90000 different 5 digit numbers.

Answer 2

first 5 digit number = 10000

last 5 digit number = 99999

it becomes an AP

10000,10001........,99999

a = 10000

d = 10001-10000 = 1

an = 99999

by using formula

an = a+(n-1)d

10000+(n-1)1 = 99999

n-1 = 99999-10000

n -1 = 89999

n = 89999+1 = 90000