Answer with explanation:
Given : Dolores each shelf to have the same number of mugs. She only wants one color of mug on each shelf.
If she has 49 blue mug and 56 red mugs then to find the greatest number she can put on each shelf, we need to find the greatest common factor of 49 and 56.
Prime factorization of 49 and 56 :-

Greatest common factor of 49 and 56 = 7
∴The greatest number she can put on each shelf =7
Now, Number of shelves for blue mug =

Number of shelves for red mug =

∴ Total shelves needed = 7+8=15