Monica's mother ate half as many cookies as Monica's father, so she ate 10 cookies / 2 = <<10/2=5>>5 cookies.
Monica's brother ate 5 cookies + 2 cookies = <<5+2=7>>7 cookies.
There were 30 cookies - 7 cookies - 5 cookies - 10 cookies = <<30-7-5-10=8>>8 cookies left for Monica.
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