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There are 8 flowers, all numbered between 10 and 20. There are 16 butterflies, 8 of which are numbered 10.
First I ordered all pairs, each having a number 10 butterfly. Next I added the pair. I then placed sum of each pair on the corresponding flower.
There were no flower with number 13, hence butterflies 10 and 3 cannot go in any flower.
No pair added to 18, so there can be no butterflies on flower 18.