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if Mary buys oranges at 5 for 2 cents and sell them at 4 for 3 cents how many must she buy and sell to make a profit of 420 cents?

profit= revenue-costs
profit=420cents=3cents/4 *N - 2cents/5 *N

N= 420/(3/4-2/5)

N=20*420/(20(3/4-2/5))=20*420/(15-8)
= 20*420/7= you do it.

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