Fast trick to calculate percentages that I always use.
If you need to find 10% of something, just move one place to the left.
Example:
10% of 100 is 10
10% of 90 is 9
10% of 823 is 82.3
10% of 120 is 12
and so on.
Now that you know how to calculate 10% of ANY number, to get 20% is to just multiply it by two.
10% of 90 is 9 so 20% is 18
10% of 823 is 82.3 so 20% is 164.6
10% of 120 is 12 so 20% is 24
Let's now try it on your problem.
10% of 80 is 8
So 20% of 80 is 16!
One more quick tip:
If you want to let's say find 23% of 80, you move two places to the left to get 1%
1% of 80 is 0.8
And now you multiply 1% with 23 to get 23%
23% of 80 is 0.8 * 23 = 18.4 and voila, now you can calculate percentages easily!