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Fourth grade math (insect experts caught 13 dragonflies and cicadas. There are 20 pairs of wings. Do you know that dragonflies and cicadas have their own?
Every dragonfly has two pairs of wings. If there are x dragonflies, there are 2x pairs of wings.

Each cicada has 1 pair of wings, and cicadas have 13-x pair of wings, that is, 13-x pair of wings.

2x+ 13-x=20,

Solution: x=7,

There are seven dragonflies and six cicadas.