Justin and Mark had 73 shortbreads altogether. After Justin ate ¹₃ of his shortbreads and Mark ate 10 shortbreads, the number of shortbreads Mark had left was thrice the number of shortbreads Justin had left. How many more shortbreads did Mark have than Justin at first?

	Justin	Mark	Total
Before	3 u	6 u + 10	73
Change	- 1 u	- 10
After	2 u
Comparing Justin and Mark in the end	1x2 = 2 u	3x2 = 6 u

The number of shortbreads that Justin had in the end is the same. Make the number of shortbreads that Justin had in the end the same.  LCM of 2 and 1 is 2.

Total number of shortbreads at first
= 3 u + 6 u + 10
= 9 u + 10

9 u + 10 = 73
9 u = 73 - 10
9 u = 63 

1 u = 63 ÷ 9 = 7

Number of shortbreads that Justin had at first
= 3 u
= 3 x 7
= 21

Number of shortbreads that Mark had at first
= 73 - 21
= 52

Number of shortbreads that Mark had more than Justin at first
= 52 - 21
= 31

Answer(s): 31