What is a formula for the nth term of the given sequence? 13, 17, 21...

1)an = 13+ 4(n - 1)
2)an= 17 + 4(n − 1)
3)an=13+4(n + 1)
4)an=13 - 4(n - 1)

Scores on a university exam are normally distributed with a mean of 78 and a standard deviation of 8. the professor teaching the class declares that a score of 70 or higher is required for a grade of at least “c.” using the 68-95-99.7 rule, what percentage of students failed to earn a grade of at least “c”?