A bookshop sells notebooks in packs of 2 and 5. At first, there were 10 times as many packs of 2 as packs of 5. After selling half of the packs of 2 and some packs of 5, Mr Goh packs 10 additional packs of 5. How many packs of 2 and 5 are sold if there are 10 times as many packs of 2 as packs of 5 and there is a total of 225 unsold notebooks?

	Packs of 2	Packs of 5
Comparing the number of packs at first	10x2 = 20 u	1x2 = 2 u
Before	2x10 = 20 u
Change 1	- 1x10 = - 10 u	- ?
Change 2		+ 10
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 notebooks left unsold	Packs of 2	Packs of 5	Total
Number	10 u	1 u
Value	2	5
Total value	20 u	5 u	25 u

Total number of notebooks left unsold
= (10 u x 2) + (1 u x 5)
= 20 u + 5 u
= 25 u

25 u = 225

1 u = 225 ÷ 25 = 9

Number of packs of 2 and 5  sold
= 10 u + (2 u - 1 u) + 10
= 10 u + 1 u + 10
= 11 u + 10
= 11 x 9 + 10
= 99 + 10
= 109

Answer(s): 109