To find out how much Li will have to earn to pay for the skateboarding set, we need to calculate the final price after all the discounts and the sales tax.
First, let's calculate the discount amount:
Discount amount = sale price * discount rate
Discount amount = $54.99 * 30% = $54.99 * 0.30 = $16.50
Next, let's calculate the sale price after the first discount:
Sale price = original price - discount amount
Sale price = $54.99 - $16.50 = $38.49
Then, let's calculate the discount amount from the coupon:
Coupon discount amount = sale price * coupon discount rate
Coupon discount amount = $38.49 * 10% = $38.49 * 0.10 = $3.85
Next, let's calculate the final sale price after the coupon discount:
Final sale price = sale price - coupon discount amount
Final sale price = $38.49 - $3.85 = $34.64
Now, let's calculate the sales tax amount:
Sales tax amount = final sale price * sales tax rate
Sales tax amount = $34.64 * 5.75% = $34.64 * 0.0575 = $1.99
Finally, let's calculate the total amount Li will have to earn to pay for the skateboarding set:
Total amount = final sale price + sales tax amount
Total amount = $34.64 + $1.99 = $36.63
Therefore, Li will have to earn $36.63 to pay for the skateboarding set.