An digit number is a positive number with exactly digits. At least nine hundred numbers are to be formed using only the three digits 2, 5 and 7. Find the smallest for which this is possible.
How many one digit numbers use only 2,5,7?
3, as you have 3 choices for the digit.
How many two digit numbers use only 2,5,7?
3*3, as you have 3 choices for the first digit, and 3 choices for the second.
(22, 25, 27, 52, 55, 57, 72, 75, 77)
Similarly, there are 3*3*3 three digit numbers that use only 2,5,7.
So..
Hello, fardeen_gen!
This one is easier than you think . . .
We can baby-talk our way through it . . .An -digit number is a positive number with exactly digits.
At least nine hundred -digit numbers are to be formed using only the three digits 2, 5 and 7.
Find the smallest for which this is possible.
There are one-digit number that can be formed.
There are two-digit numbers that can be formed.
There are three-digit numbers that can be formed.
There are four-digit numbers that can be formed.
There are five-digit numbers than can be formed.
There are six-digit numbers than can be formed.
There are seven-digit numbers than can be formed.
Therefore: .
I'll let someone else show you a more sophisticated method.