Question 1061419
Let c be the cost per person for the expedition, and p be the amount of people originally expected to go. Then:
cp=900 
p=900/c
and
900/(c+15)=p-3
900/(c+15)=900/c-3
900c=900c+13500-3cē-45c
3cē+45c-13500=0
cē+15c-4500=0
(c+75)(c-60)=0
c=60 or -75
Throwing out the negative number, we get the original cost as $60 per person, which would have been split between 15 people. ☺☺☺☺