Final answer:
The regular price of a shirt was $15. Natalie bought it on sale for $13.50. After including tax, the final price of the shirt was $14.31.
Step-by-step explanation:
To find the total cost of the shirt after tax, we need to first find the sale price by multiplying the regular price ($15) by 0.9. This gives us $13.50. Next, we multiply the sale price by 1.06 to include the tax. This gives us a final price of $14.31. Therefore, Natalie paid $14.31 for the shirt after tax.