So,
There are 5 times as many yellow labs (call them y) as terriers (call them t).
5t = y
There are 18 dogs total.
t + y = 18
Substitute 5t for y in the second equation.
t + 5t = 18
6t = 18
Divide both sides by 6.
t = 3
There are three terriers and 15 yellow labs.
So the system of equations would be:
5t = y
t + y = 18
As for the model, you can have a circle represent the number of terriers. Since there are 5 times as many yellow labs as terriers, you will have to have five more circles. Adding them up, you will have 6 of the circles, or 6 times circle. The total is 18, so 6 of the circles is 18. 6 times what is 18? 3. Therefore the circle, and therefore the number of terriers, is 3.