Question 13... Initially you need to find the sum of all of his test scores. To do this, you do $\displaystyle 84\times 6$ to undo the dividing by 6. Then divide this number (which is $\displaystyle 504$, by the way) by the 7 to obtain 72 as your answer.

Question 14 is similar to the problem above really. Multiply the juniors' average score of 80 by the 35 to get 2800, and multiply the seniors' average score of 70 by the 15 to get 1050. Add these two together (to obtain the total of all scores) to get 3850. Then finally, to get the total average, divide by the total number of students (50) to get 77 as your answer.

Hope this helps.