assessment test in a few days in some help!

Need Help! Would like To know How to solve these..... Would like to know step by step(formula)

Numerical Skills/Prealgebra

13. Adam tried to compute the average of his 7 test scores. He mistakenly divided the correct sum of all of his test scores by 6, which yielded 84. What is Adam’s correct average test score?

A. 70

B. 72

C. 84

D. 96

E. 98

14. A total of 50 juniors and seniors were given a mathematics test. The 35 juniors attained an average score of 80 while the 15 seniors attained an average of 70. What was the average score for all 50 students who took the test?

A. 73

B. 75

C. 76

D. 77

E. 78