I wasn't sure whether marked up meant increased or like discounted so ill show you step by step. 10% is $5. 20% would be $10 because it is double the price of 10%. Therefore if you meant it is 20% more than the original it would be $60. If you meant it would be 20% less is would be $40. Sorry its not much help but i wanted to do the best i could.
I think you meant that the overall price is only 20% of the original price, so I will do that. 20% of 100 is 20, and 50 is half of 100. that means that 20% of 100 is double 20% of 50. Since 20% of 100 is 20, and 20% of 50 is half that, 20% of 50 is 1/2 of 20, which is 10. So 20% of $50 is $10
