A stationery booth 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 Thia packs 7 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 399 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 |
|
+ 7 |
| 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 = 399
1 u = 399 ÷ 57 = 7
Number of packs of 7 sold
= (4 u - 1 u) + 7
= 3 u + 7
= 3 x 7 + 7
= 21 + 7
= 28
Answer(s): 28
