If 300 jellybeans cost you x dollars. How many jellybeans can you purchase for 50 cents at the same rate?
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The price P of 1 jellybean is x/300 dollars
so for 0.5 dollars you can buy 0.5/P jellybeans = 150/x
so for 0.5 dollars you can buy 0.5/P jellybeans = 150/x
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300/x*50/100=150/x