There are spiders, dragonflies and cicadas. There are three kinds of animals *** 18. Each spider has eight legs and no wings. Each dragonfly has six legs and two pairs of wings. Each cicada has six legs, 1 pair of wings. How many dragonflies do you have?

Solution:

Because dragonflies and Zen have the same number of legs.

So treat them as a whole.

Suppose they are all spiders.

Then * * * has legs 8× 18= 144.

Then it is more than the actual144-128 =16.

Dragonflies and cicadas *** 16÷(8-2)=8.

Suppose all eight are dragonflies.

Then a * * * has wings 8×2= 16.

More than the actual 16- 13=3 pairs.

So there are 3 cicadas (2- 1) = 3 cicadas.

Dragonflies have 8-3=5.

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