Answer
You will pay $0.20 (20 cents) more on a $20 purchase.
Step-by-step explanation
To find a percentage of any number we just need to multiply the number by the percentage and divide by 100
First, we'll find the 7% of $20:

Next, we'll find the 8% of $20:

Finally, we just need to subtract $1.40 form $1.60:

We can conclude that after the increasing of sales tax from 7% to 8%, you will pay $0.20 (20 cents) more on a $20 purchase.