A group of people, cats, and birds has seventy legs, thirty heads, and twenty
tails. How many cats are among this group? (Assume all birds have two legs and a tail.) Choose the correct choice from the following options.
A) 0  B) 5  C) 10  D) 15  E) 20
Write P, C, and B for the number of people, cats, and birds respectively, so that   we have   2P + 4C + 2B = 70; P + C + B = 30; C + B = 20:   Comparing the last two equations we obtain P = 10, and the rst equation simplies to   4C + 2B = 50, or 2C + B = 25. Therefore,   C = (2C + 5) / (C + 5) = 25 / 5= 
<gadget id="calculator">25 / 5</gadget>
<output>5</output>
5:   Alternatively,   C= 1   2   2P + 4C + 25   P + C + 5   = 35 - 30= 
<gadget id="calculator">35 - 30</gadget>
<output>5</output>
5:   correct answer 5
<result>B</result>
A group of people, cats, and birds has seventy legs, thirty heads, and twenty
tails. How many cats are among this group? (Assume all birds have two legs and a tail.)