Math Problem (Level 1) : List price discounted (C3)

If you sell a product of list price 100 yen at 20% discount, it is the loss of 10 yen. 

How much is the cost price of this product?

Answer

90 yen

Solution

The 20% discount of 100 yen is this for 100 yen × (1 - 0.2) = 80 yen. 

As for the cost price, it is 80 yen + 10 yen = 90 yen to lose 10 yen.