Kenny used 38 / 4 = <<38/4=9.5>>9.5 mushrooms on his pizza.
Karla used 42 / 4 = <<42/4=10.5>>10.5 mushrooms on her pizza.
Together, the twins used 9.5 + 10.5 = <<9.5+10.5=20>>20 mushrooms.
There are 8 / 4 = <<8/4=2>>2 mushrooms left on the cutting board.
GiGi cut up 20 + 2 = <<20+2=22>>22 mushrooms at the beginning.
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