If all three have six legs, then a * * should have 18× 6 = 108 legs.

But in fact, there are118-108 =10 more legs than calculated.

This is because each spider has 8-6 more legs than 6 = 2.

This means that there are 10 ÷ 2 = 5 spiders.

There are 18-5 = 13 dragonflies and cicadas.

Similarly, if 13 are all dragonflies, there should be 13× 2 = 26 pairs of wings.

But there are actually 26-20 = 6 pairs missing.

This is because each cicada has one less pair of wings than two, so there are 6 cicadas +0 = 6.

13-6 = 7 There are 7 dragonflies.