This task is from Tools 4 NC Teachers.  Students explore the relationship between numbers and look for patterns. This is remixable.

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Farmer Brown has some cows and chickens in his barn.  Altogether, his animals have 40 legs.  How many of them are cows, and how many are chickens?

Create a table and line graph to explore the different possibilities.

What patterns do you notice? How did the graph help you to see the patterns?

Variation:  Give a specific number of animals.  How many were chickens and how and cows?