The answer should be C. 8
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
I hope this helps you understand how to do the problem.
The answer is 18 . Because 36 divided by 2 is 18
The answer is 1/64 because you do 3/8 ÷ 24/1 then you
flip and it will be 3/8 ÷ 1/24 you can cross cancel
24 go's into three 8 times so the answer is 1/64s.
Hope I helped!
=)