In a zoo, there are rabbits and pigeons. If heads are counted, there are 340 heads and if legs are counted there are 1060 legs. How many pigeons are there?
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While this problem can be easily done using a system of equations (as Krishna's solution states), this problem is also solvable by a bit of slick thinking.
We first assume that the zoo is all pigeons; that is, we assume that there are 3 4 0 pigeons. Clearly, this will fail to fulfill the legs requirement; in this case, we're 1 0 8 0 − ( 2 × 3 4 0 ) = 3 8 0 legs short.
We then note that by replacing a pigeon with a rabbit, we essentially "add" two legs. Thus, to fulfill the legs requirement, we need to replace 3 8 0 / 2 = 1 9 0 pigeons with rabbits. Thus, we have 1 9 0 rabbits and 1 5 0 pigeons.