You can put this solution on YOUR website!
Answer for your question no 1
Cost of the cabin is 600000$
Let the no of people be X , now they have mentioned that if 5 more people join the sharing each will save 6000$
So the amount contributed by each of X members is obviously > 6000$
And for that the number of contributors should also be less.
Now take 2 values 20 & 25
if x is 20 then contribution is 600000/20 = 30,000
and x+5 that is 25, then 600000/25 = 24,000
thus 30000-24000= 6000$
This fits our requirement.
Ans 25 people
Another method is
Let no of contributors = x
and contribution made = y
thus x*y = 600000
Now solve to get values for x and y
This will take much longer time than the 1st one.