Given:
Mai sells boxes of cookies for $10 each.
Deandre sells boxes of cookies for $8 each.
Let they sold for the amount of $40 each. [40 is the LCM of 8 &10]
Let x be the number of boxes sold by Mai and y be the number of boxes sold by Deandre.
Dollar amount each of them sold is $40.
Number of boxes sold by Mai is 4.
Number of boxes sold by Deandre is 5.