First find the total number of minnows by dividing the number of red minnows by the percentage of minnows that are red: 20 minnows / 40% = 50 minnows

Then subtract the percentages of red- and green-bellied minnows to find the percentage of white-bellied minnows: 100% - 40% - 30% = 30%

Then multiply the total number of minnows by the percentage that have white bellies: 30% * 50 minnows = <<30*.01*50=15>>15 minnows

# Answer

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<|im_start|>user
Mary is counting the number of minnows in the pond. 40% of the minnows have red bellies, 30% have green bellies and the rest have white bellies. If 20 minnows have red bellies, how many minnows have white bellies?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
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15
Mary is counting the number of minnows in the pond. 40% of the minnows have red bellies, 30% have green bellies and the rest have white bellies. If 20 minnows have red bellies, how many minnows have white bellies?
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