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.