Your grades in Math for the first and second trimesters are 74 and 75, respectively. What grade in the third trimester will make your average grade at least 80?

Solution:
Average grade for 3 semesters should be 80.
Average = total grades for 3 sems / 3
Let the grade for 3rd trimester be x.

So, 
 or 

So you need to score  in the 3rd trimester to get an average of 80.

Hope this helps :)

Cheers,
Ramki.