How to simplify 12ab^2÷15a^2 in rational expression

How do you solve a question like this?

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”?

In the sixth grade 13 out of 20 students have a dog if there are 152 sixth grade students how many of them have a dog