Question 1173103
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If a formal algebraic solution is not required, this kind of problem can be solved using logical reasoning and simple mental arithmetic.<br>
(1) all 21 animals being ducks would mean 2(21) = 42 legs; that is 72-42=30 less than the actual number.
(2) Each cow has 2 more legs than each duck.
(3) The number of cows needed to make those additional 30 legs is 30/2 = 15.
(4) 15 cows means 21-15 = 6 ducks.<br>
ANSWER: 6 ducks<br>