The shoes have an original price of $40 but are $10 off, this means that the sale this week is $30.
An additional 20% off means that it is basically 80% of the shoe being sold this week. So
Since the shoes he is going to buy are $24 dollars out of the $40 original price. The price dropped $16.
What percent of 40 is 16? You can convert it into a fraction by dividing 16 by 40. So
Since is easily recognized as 40%. The total percent discount is 40%