Thank you so much, much clearer now, but the problem is during PSLE exam, which is only last one hour plus only, I don’t think i have enough time to do this method, it time consuming, it will eat up the time to finish other questions to do… thank you so much

Yes. If use combination will kill time.
Likewise, you need to think this way. Only 2 hour at dance studio, each time 5 girls, total spend in studio 5×2=10 hours.
8 girls spend 10 hours.
1 girls spend 10/8= 1.25 hours = 1hr15mins

Yes like AC said, you should think of it like a question on average.

The only part is you need to consider the TOTAL number of hours spent dancing by the girls

The intervals that they change isn't really fixed also.

You can also make it like every 1 minute they change and eventually the sequence/order will repeat every 8 minutes.

Everyone will still get 75 minutes of dancing in the end.

To understand the concept, may be can start from easy questions.
Example: 4 students book badminton court for 1 hour. Each time 2 students play.
By intuitive, 1st 2 students played ½ hour then next 2 students played another ½ hours.
Each of them played ½ hour

By average...
2 students only play in 1 hour, total hours = 2×1 hour = 2 hours

4 students → 2 hr
1 student → 2/4 = ½ hour.

There is an alternative way to think about this question as well, which is fractions.

I'll give you a simpler example first.

① If there are 2 hours available to to play on 1 computer and there is only 1 girl, this girl can play for the full 2 hours.

②However, if there are 2 girls now and they have to share the time equally, then each girl can only play for ½ the total time.

2 hours × ½ = 1 hour

③ If there are 3 girls now then each girl can only play for ⅓ of the 2 hours.

2 hours × ⅓ = ⅔ hours = 40 minutes.


④If there are 2 computers now but still 3 girls, then ⅔ of the time.

2 hours × ⅔ = 1⅓ hours = 1h 20 min

⑤ If there are 4 computers now but 7 girls, then 4/7 of the time.


So, if there are 5 computers but 8 girls, each girl only gets to play ⅝ of the time.


2 hours × ⅝ = 5/4 hours = 1¼ hours

= 1 hour 15 min

@J Thanks for your super good examples and explanation. :)

Your explanation and diagram is great as well AC

Thank you so much, all the explaining is good, much clearer, thank you all ( J) and (AC). Thank you so much, thank all of you, clear my doubts