A box of cupcakes cost $15 while a box of shortbreads cost $16. Each box contained either 9 cupcakes or 4 shortbreads. Tammy bought the same number of cupcakes and shortbreads. She paid $840 more for the shortbreads. How many boxes of cupcakes did she buy?
To make the number of cupcakes and shortbreads that Tammy bought the same, find the LCM of 9 and 4.

LCM of 9 and 4 = 36

Cost of 1 set of 36 cupcakes
= 15 x 4
= $60

Cost of 1 set of 36 shortbreads
= 16 x 9
= $144

Amount that Tammy paid more for 1 set of 36 cupcakes than 1 set of 36 shortbreads
= 144 - 60
= $84

Number of sets of $84
= 840 ÷ 84
= 10

1 set of $84 → 4 boxes of cupcakes

10 sets of $84 → 10 x 4 = 40 boxes of cupcakes

Number of boxes of cupcakes that Tammy bought = 40

Answer(s): 40