Cost of the computer before tax = $1500
Percentage of sales tax taken = 7%
Then
The amount of sales tax taken = (7/100) * 1500
 = 7 * 15
 = 105 dollars
So the total amount of sales tax taken is $105
Then
The total cost of the computer after adding sales tax = (1500 + 105) dollars
 = 1605 dollars
So $1605 is the total cost of the computer after adding 7% tax. I hope the procedure is clear enough for you to understand.