At a school camp, 30% of the students were boys. When 87 students left the camp after the second day, half the original number of boys were left behind and the number of girls decreased by 20%. How many girls were at the camp in the beginning.

Answer 1

number of boys --- 3x
number of girls ---- 7x

total campers = 10x

after leaving:
total campers = 10x-87
number of boys = 3x/2
number of girls = (8/10)(7x) = 28x/5

3x/2 + 28x/5 = 10x - 87
times 10
15x + 56x = 100x - 870
-29x = -870
x = 30

number of girls at start = 7x = 210

check:
at start:

boys -- 90
girls -- 210
total -- 300

after the wimps left:
boys = 45
girls = .8(210) = 168
total = 45+168 = 213
which is the same as 300-87 = 213

all looks good