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Bugs buster loves collecting bugs in his tank. He has 300 spiders and millipedes in his tank, of which 10% are spiders. How many millipedes must he remove, such that 20% of the bugs that are left will be spiders?

 Jul 2, 2020
 #1
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There are 300 spiders and millipedes, of which 10% are spiders:  0.10 · 300  =  30.

He has 30 spiders and 270 millipedes.

 

He wants a group of millipedes and spiders to contain 30 spiders and these 30 spiders 

are 20% of the group.

 

Let  x  be the total number of millipedes and spiders:  0.20 · x  =  30

Dividing both sides by 0.20:                                                    x  =  30 / 0.20  =  150.

 

He'll end with 30 spiders and 120 millipedes.

 

He'll need to remove  270 - 120  =  150  millipedes.

 Jul 2, 2020

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