"For 6 dollars you can buy 15 candies. How much money do you need to buy 35 candies?"
It is important to remember that on both sides of "equal" sign numbers at certain position represent same "type". In this example first number on left side is 6 and on right side is "n" and we can see in text that both of them represent money, while other 2 numbers represent other type (in this case, candies).
