A quiz - a zoo has birds and mammals , need to work out the numbers?!

A quiz - a zoo has birds and mammals , need to work out the numbers?

a zoo, has just mammals & birds. When asked how many of each they have, the zoo keeper said he had 54 animals with 140 feet total between them all. Assuming all the animals have all the feet they should have, how many birds & how many mammals are there?

3 weeks ago
How did you arrive at the answer ?

Let X be the number of mammals.
Let Y be the number of birds.
Then X+Y= 54 (the total number of animals)
This is our first equation.

Each mammal has 4 feet. Each bird has 2 feet.
So total number of mammal feet (4 times X) + total number of bird feet (2 times Y) = 140 (the total number of animal feet)
i.e. 4X + 2Y = 140 or, if we divide both sides by 2,
2X + Y = 70
This is our second equation.

Now, if we subtract our first equation from our second equation we get X = (70 - 54) = 16
Looking back to our first equation we can see that if X = 16
then Y = (54 - 16) = 38
So there are 16 mammals and 38 birds.
Now that wasn't difficult, was it?