Answer:
90
Explanation:
Using inequalities we can understand this problem.
First call X the number of products in the original batch
Then X/6 is the number of muffins in each tray before putting the croissants
Finally, X/6+5 are the products in each tray counting the croissants, and this quantity at least should be 20, it means:

Isolating X:

The least possible number of muffins in the baker's original batch is 90.