Common sum with number System for class 8

1.Write all possible numbers using the digits 3,6 and 8 without repeating of digits.

Solution:
The numbers are:

Question: Do it yourself for 2,4 and 9

2. How many whole numbers have two digits.

Solution:The two digits whole numbers are from 10 to 99. e.g. 10,11,12……99.
Therefore the two digit numbers= 99-(10-1)
=99-9
=90

Note:Its general formula is : For two digits number = two digits highest number-(two digits lowest number-1)

3.How many whole numbers have three digits.

Solution:The two digits whole numbers are from 100 to 999. e.g. 100,101,102……999.
We will apply the same formula by changing the numbers of digits only.
Therefore the three digit numbers= 999-(100-1)
=999-99
=900
Used formula is :three digits highest number-(three digit lowest number-1)

Solution:The four digits largest number = the digits in descending orders (9,7,3,0)
=9730
The four digits smallest number = the digits in ascending orders (0,3,7,9) but we have to keep smallest digit 3 at the start to make it four digit smallest number otherwise it will become three digits number if it is started with 0.
=3079

5.Find the difference between smallest 6-digit number and and greatest 5-digit number.

Solution:The smallest 6-digit number=100000
The greatest 5-digit number=99999
difference= 100000-99999
=1 Ans.