A petting zoo has a combined total of 16 frogs and chickens with a total of 42 legs. What is the difference between the number of frogs and the number of chickens?

Convert the word problem into a system of linear equations and solve for each variable in order to calculate the difference. Let f equal the number of frogs and c equal the number of chickens. Each frog has 4 legs and each chicken has 2 legs.
f + c = 16
4f + 2c = 42
Use any method to solve the system. By multiplying the first equation by -2 and combining the equations, the number of frogs can be found:
2f = 10, so there are 5 frogs. 5 + c = 16, so there are 11 chickens.
The difference is then 11 - 5 = 6.