Credit cards place a three-digit security code on the back of cards. what is the probability that a code starts with the number 7

So the numbers on the credit card can start with any single-digit number

So it could start with;
0
1
2
3
4
5
6
7
8
Or 9

We are looking for the odds of it starting with a 7

There is only one 7, so the numerator is 1
There are then nine other possible numbers it could be, and counting the number 7, there is a total of ten different possibilities, so 10 is the denominator

So the odds of the card starting with a 7 is
1/10