From the information we have, we know that the total amount of money she will pay for all the magnets amounts to 16.80 dollars.
We also know that the price or cost of a single magnet is 0.80 dollars each.
Now we simply divide her total bill with the price of one magnet, so:
16.80/0.80 = 21
Therefore the total number of magnets she is buying is 21
We can make the calculation also by removing the decimal points This makes it a bit more easier for calculation. We multiply each by 100 to eliminate the two decimal places:
16.80 * 100 = 1680
0.80 * 100 = 80
Now we can easily divide these numbers and we end up getting the same answer, so:
1680/80 = 21