A stationery booth sells staplers in packs of 2 and 4. At first, there were 10 times as many packs of 2 as packs of 4. After selling half of the packs of 2 and some packs of 4, Mr Chiam packs 12 additional packs of 4. How many packs of 2 and 4 are sold if there are 10 times as many packs of 2 as packs of 4 and there is a total of 120 unsold staplers?

	Packs of 2	Packs of 4
Comparing the number of packs at first	10x2 = 20 u	1x2 = 2 u
Before	2x10 = 20 u
Change 1	- 1x10 = - 10 u	- ?
Change 2		+ 12
After	1x10 = 10 u
Comparing the number of packs in the end	10 u	1 u

The number of packs of 2 in the end is repeated. Make the number of packs of 2 in the end the same. LCM of 1 and 10 is 10.

The number of packs of 2 at first is repeated. Make the number of packs of 2 at first the same. LCM of 20 and 10 is 20.

Number of staplers left unsold	Packs of 2	Packs of 4	Total
Number	10 u	1 u
Value	2	4
Total value	20 u	4 u	24 u

Total number of staplers left unsold
= (10 u x 2) + (1 u x 4)
= 20 u + 4 u
= 24 u

24 u = 120

1 u = 120 ÷ 24 = 5

Number of packs of 2 and 4  sold
= 10 u + (2 u - 1 u) + 12
= 10 u + 1 u + 12
= 11 u + 12
= 11 x 5 + 12
= 55 + 12
= 67

Answer(s): 67