18 animals in a pet store. some are birds and some are dogs. there are 50 legs in all. how many birds and how many dogs are there

A bird has only 2 legs, and a dog has 4 legs.
x=number of birds
y=number of dogs

We can susggest this system of equations:

x+y=18
2x+4y=50

We can solve this system of equations by substitution method.
x+y=18        ⇒      y=18-x

2x+4(18-x)=50
2x+72-4x=50
-2x=50-72
-2x=-22
x=-22/-2
x=11

y=18-x
y=18-11
y=7

we have 11 birds and 7 dogs.

The american colonies would have been ruled by a king.