I only need help with 2,5, and 12

Which choice represents the sample space ,s for this event

There are 45 questions on your math exam. you answered 810 of them correctly. how many questions did you answer correctly?

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.