Question
How many four-digit numbers are there in all?
✓
Answer
There are $10$ digits i.e., $0, 1, 2, 3, 4, 5, 6, 7, 8, 9.$
We cannot use $‘0’$ at thousand’s place.
So, we can use only $9$ digits at thousand’s place.
Also, we can use $10$ digits at hundred’s, $10$ digits at ten’s and $10$ digits at unit’s place.
So, total numbers of four-digit numbers $= 9 \times 10 \times 10 \times 10 = 9000$
We cannot use $‘0’$ at thousand’s place.
So, we can use only $9$ digits at thousand’s place.
Also, we can use $10$ digits at hundred’s, $10$ digits at ten’s and $10$ digits at unit’s place.
So, total numbers of four-digit numbers $= 9 \times 10 \times 10 \times 10 = 9000$
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