How many of each animal are there, and how do I work it out?: Penguins 2 legs; huskies 4 legs; 200 legs and 72 heads.

If we'll note the number of penguins by a and the number
of huskies by b, we'll get:

a+b = 72
(1)

We also could express the number of penguins and
huskies, depending on the number of legs:

2a + 4b = 200
(2)

Now, all we have to do is to solve the simultaneous
equations (1) and (2).

We'll solve the system using
elimination method. For this reason, we'll multiply (1) by
-2.

 -2(a+b) = -2*72

-2a - 2b
= -144 (3)

We'll add (3) to (2) and we'll
get:

-2a - 2b + 2a + 4b = -144 +
200

We'll eliminate like
terms:

2b = 56

We'll divide by
2:

b =
28

We'll substitute b in
(1):

a+b = 72

a + 28 =
72

We'll subtract 28 both
sides:

a =
72-28

a =
44

So, we'll get 44 penguins
and 28 huskies.