An example using reduce and map functions from JavaScript

reduce_and_map.js

const students_grades = [
  [7.0, 8.0, 6.0],
  [10.0, 10.0, 10.0],
  [5.0, 9.0, 10.0]
]

const reducer = (exams) => {

  let starter = 0
  let number_of_exams = 3

  result = exams.reduce(
    somatory, starter
  );
  return result / number_of_exams;
}

const somatory = (accumulator, grade) => {
  return accumulator + grade;
}

average = students_grades.map(reducer);

console.log(average);
// Output [7, 10, 8]