Question

9 students can make a poster in 10 hours. How many students should join them so that they all together can make this poster in 6 hours

Answer 1

If 9 students make the poster in 10 hours
Then it would take 1 student 90 hours to make 1 poster
Therefore you do 90 hours/6hours to get 15
So you'd need 15 students to make the poster in 6 hours
Which means 6 more students would need to join them

Answer 2

Answering your question on "9 students can make a poster in 10 hours. How many students should join them so that they all together can make this poster in 6 hours" its 13 students