docs: add instructions for calculating grade intervals (#908)

fixes #681

Author: vaeng

exercises/concept/making-the-grade/.docs/instructions.md

@@ -80,6 +80,15 @@ letter_grades(88);
 // => {41, 53, 65, 77}
 ```
 
+### How to calculate the intervals
+
+The score needed to avoid failing is always 41.
+The remaining score range, from 41 to the highest score, should be divided into four equal intervals, one for each letter grade.
+
+- Subtract 40 from the highest score to get the total range of scores that qualify for letter grades (A to D).
+- Divide this total range by 4 to get the size of each grade interval.
+- Add this interval size to 40 repeatedly to calculate the lower bounds for each letter grade.
+
 ## 5. Matching Names to Scores
 
 You have exam scores in descending order, and the respective student names (sorted in the order of their exam scores).