A box of cookies cost $19 while a box of shortbreads cost $13. Each box contained either 9 cookies or 2 shortbreads. Mary bought the same number of cookies and shortbreads. She paid $553 more for the shortbreads. How many boxes of cookies did she buy?
To make the number of cookies and shortbreads that Mary bought the same, find the LCM of 9 and 2.

LCM of 9 and 2 = 18

Cost of 1 set of 18 cookies
= 19 x 2
= $38

Cost of 1 set of 18 shortbreads
= 13 x 9
= $117

Amount that Mary paid more for 1 set of 18 cookies than 1 set of 18 shortbreads
= 117 - 38
= $79

Number of sets of $79
= 553 ÷ 79
= 7

1 set of $79 → 2 boxes of cookies

7 sets of $79 → 7 x 2 = 14 boxes of cookies

Number of boxes of cookies that Mary bought = 14

Answer(s): 14