A bookstore sells pencils in packs of 3 and 4. At first, there were 10 times as many packs of 3 as packs of 4. After selling half of the packs of 3 and some packs of 4, Mr Long packs 18 additional packs of 4. How many packs of 3 are sold if there are 10 times as many packs of 3 as packs of 4 and there is a total of 204 unsold pencils?

	Packs of 3	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		+ 18
After	1x10 = 10 u
Comparing the number of packs in the end	10 u	1 u

The number of packs of 3 in the end is repeated. Make the number of packs of 3 in the end the same. LCM of 1 and 10 is 10.

The number of packs of 3 at first is repeated. Make the number of packs of 3 at first the same. LCM of 20 and 10 is 20.

Number of pencils left unsold	Packs of 3	Packs of 4	Total
Number	10 u	1 u
Value	3	4
Total value	30 u	4 u	34 u

Total number of pencils left unsold
= (10 u x 3) + (1 u x 4)
= 30 u + 4 u
= 34 u

34 u = 204

1 u = 204 ÷ 34 = 6

Number of packs of 4 sold
= (2 u - 1 u) + 18
= 1 u + 18
= 1 x 6 + 18
= 6 + 18
= 24

Answer(s): 24