math
posted by trevor .
the trainers took a count of the animals at the zoo. there are both birds and four legged mammals at the zoo. the trainers counted 550 feet and 155 animals on their first trip through. How many birds and mammals are there?

x=number of 2 legged birds
y=number of 4 legged mammals
2x+4y=550
if there are a total of 155 animals and you know the number of 2 legged birds, you could subtract the two and find the number of 4 legged mammals
therefore, let y=155x
substitution:
2x+4(155x)=550
solve for x. that is the number of 2 legged birds!
then plug x into y=155x and that will be the number of four legged mammals 
15