Answer:
Don has to sell 61 marbles.
Explanation:
This question is solved by a system of equations.
I am going to say that:
Don has to sell x marbles.
Mark has to sell y marbles.
The total number of marbles is 75:
This means that:

Since we want to find x.

Don has 5 more than 4 times the number of marbles Mark has
This means that:

Since






Don has to sell 61 marbles.