First measure is simply for convenience, to avoid writing the same summation multiple times.
Second measure: First, we find index visible in a current row and save it in a variable. Then, we find the initial amount (amount where index = 1), because we need it as a starting point. ALL is necessary to ignore filters applied to the row. Then, using a similar pattern, we calculate accumulated reductions between current and the initial rows (for example, for step D, we need to sum amounts for indexes (4, 3, 2)) Finally, the desired result is simply initial amount - accumulated reductions.
Third measure: Using very similar technique, we first find Accumulated Difference for the current row, then for the previous row. The only nuance here is to test previous row for the starting conditions (if it's the initial row with index 1, don't calculate previous values). Once current and previous differences are known, we simply divide them.
If you are preparing for the Power BI certification exam, then take up this Power BI training by Intellipaat that offers instructor-led training, hands-on projects, and certification.