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

The probability is  since the maximum value of digits allowed in the ones place is 9 (10 Counting 0, but it wont start with 0) and so since 7 is one out of nine the answer is truly

P.S. For decimal the answer is: 0.111111 and for Percentage it is 11.1111%

The decimal is behind the last number in a percent. So, you go two places to the left. That would make it 3.52 as a decimal of 352%