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word problem trying to set up a formula to find the answer

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Mental math: Assuming all are birds, you would have 170*2 = 340 feet. Now you have 402-340 = 62 extra feet. That means you have 62/2 = 31 rabbits because switching each bird to a rabbit you get two extra feet.
Answer: 31 rabbits and 170-31 = 139 birds.


Thanks so much for putting it into simple form for me. Sometimes we try to over think the obvious.Appreciate your help
Let's say there are rabbits and birds
Since there are 170 heads and each bird and rabbit has one head, we can write the following first equation
r + b = 170  ----------------let's call this equation 1
Also, each rabbit has 4 legs, so rabbits have a total of 4 * = 4 legs
Each bird has 2 legs, so b birds have a total of 2legs
The total number of legs from the above accounting is 4r + 2b legs
But we know that the total legs observed are 402.  With this information we form the second equation below
4r + 2b = 402 ----------------let's call this equation 2
solving for the variables using equations 1 and 2, we get our solution of 31 rabbits and 139 birds