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  Number & Math Play

Spiders and Insects

How many insects were playing? How many spiders?

A 8 spiders, 12 insects

You know there are 20 animals, and more insects. If all were insects, there would be 120 legs. But there are 136, which means that 16 additional legs must belong to spiders. Since each spider gets 2 of those 16 legs, divide 16 by 2 to get 8 spiders. Subtract 8 from 20 to get 12 insects.

STRATEGY: Work backwards.

Source: Barnes and Noble, 100 Numerical Games, Pierre Berloquin