Hope you are well.
The solution 640 mn is correct. Let me explain:
If something has 15% margin it means that Margin / Sales = 15%
Therefore (Sales - Cost) / Sales = 15%
Also, Cost = (100%-15%)*Sales
Thus, if you know that Cost = 544mn, the equation you want to solve is:
544 = (100%-15%)*Sales; Sales = 544 / 85%; Sales = 640 mn
This way you can validate also that 544 / 640 = 85%; which leads to 15% Margin as you were told on the question.
In the response you show you are calculating an increment which is not correct:
544 / 626 = ~87%, which leads to a Margin of ~13%, which is not correct.
Is then clear? Basically you need to take into account that Margin is part of the sales, not an increment.
Let me know if you need something else!