Question 159912
Suppose you own a card shop. You buy one line of cards at a rate of 4 cards for $5. You plan to sell the cards at a rate of 3 cards for $5. How many cards must you sell in order to make a profit of $100.
:
Let x = no. of cards required to make $100 profit
:
Cost = (5/4)x
Revenue = (5/3)x
:
 Revenue - cost = Profit
(5/3)x - (5/4)x = 100
Multiply equation by 12 to get rid of the denominators; results:
4(5x) - 3(5x) = 12(100)
20x - 15x  = 1200
:
5x = 1200
x = {{{1200/5}}}
x = 240 cards need to be sold
:
:
Check solution: (5/3)240 - (5/4)240 = 100