math 1
posted by HELP HELP HELP MNHS .
The total outing cost for a group of people amounted to Php 8000. When an additional 4 persons decided to join the outing, the cost per person decreased by 100. How many people actually joined the trip?

let number of people originally be n
cost per person = 8000/n
new count = n+4
new cost = 8000/(n+4)
8000/n  (8000/(n+4) = 100
times n(n+4)
8000(n+4)  8000n = 100n(n+4)
expand after dividing by 100
80n + 320  80n = n^2 + 4n
n^2 + 4n  320 = 0
(n+20)(n16) = 0
n = 16 or n=20 , but clearly n > 0
so originally 16 people had signed up, but 4 more joined,
So there were 20 people on the trip
check:
original cost = 8000/16 = 500
new cost = 8000/20 = 400
YUP! 
thanks Reiny, hope i am as intelligent as you.... :)