Jan bought a blazer for $39.90. the price of the blazer was $38. what sales-tax rate was she charged for this blazer?

First find the amount of sales tax by Subtracting 39.90 - 38 = 1.90
The amount of sales tax is $1.90. Since sales tax is found by multiplying the tax rate (in decimal form) times the original cost, we have this equation:
Sales tax = rate * original cost.
We know that sales tax is $1.90 and the original cost is $38.
Plug these numbers into the equation:
1.90 = r * 38
Solve for r by dividing both sides by 38:
r = 0.05
This is in decimal form, so convert it to percent form by moving the decimal 2 places to the right.
Sales Tax rate = 5%

38 dollars is your answer.