Answer: $702.39
Step-by-step explanation: To find the total cost of the laptop with sales tax included, we need to add the sales tax to the listed price.
First, we need to calculate the amount of sales tax.
Sales tax = 9.75% of 639.99
Sales tax = 0.0975 x 639.99
Sales tax = 62.40
Now we can find the total cost of the laptop with sales tax included:
Total cost = listed price + sales tax
Total cost = 639.99 + 62.40
Total cost = 702.39
Therefore, the total cost of the laptop with sales tax included is $702.39.