Problem 3. Philip bought 100 candies at a price of 5 candies for 7 denars, and then sold them all at a price of 2 candies for 3 denars. How many denars did Philip earn?
Solution. Philip bought 100 candies for (100:5) $\cdot$7 = 140 denars, and for them, he received (100:2)$\cdot$3=150 denars. According to this, Philip earned $150-140=10$ denars.