A stationery kiosk sells markers in packs of 5 and 7. At first, there were 5 times as many packs of 5 as packs of 7. After selling half of the packs of 5 and some packs of 7, Mr Tang packs 16 additional packs of 7. How many packs of 5 are sold if there are 10 times as many packs of 5 as packs of 7 and there is a total of 570 unsold markers?

	Packs of 5	Packs of 7
Comparing the number of packs at first	5x4 = 20 u	1x4 = 4 u
Before	2x10 = 20 u
Change 1	- 1x10 = - 10 u	- ?
Change 2		+ 16
After	1x10 = 10 u
Comparing the number of packs in the end	10 u	1 u

The number of packs of 5 in the end is repeated. Make the number of packs of 5 in the end the same. LCM of 1 and 10 is 10.

The number of packs of 5 at first is repeated. Make the number of packs of 5 at first the same. LCM of 20 and 5 is 20.

Number of markers left unsold	Packs of 5	Packs of 7	Total
Number	10 u	1 u
Value	5	7
Total value	50 u	7 u	57 u

Total number of markers left unsold
= (10 u x 5) + (1 u x 7)
= 50 u + 7 u
= 57 u

57 u = 570

1 u = 570 ÷ 57 = 10

Number of packs of 7 sold
= (4 u - 1 u) + 16
= 3 u + 16
= 3 x 10 + 16
= 30 + 16
= 46

Answer(s): 46