Question

Andrea wanted to calculate 5% of 300, so she multiplied 300 by 0.5 (300 x 0.5).

Answer 1

This is because Andrea converted the Percentage wrong.
to Convert a Percent to a Decimal, she should have divided by 100
so  5 divided by 100 = 0.05  [not 0.5]
Andrea should have multiplied 300 by 0.05 instead to get the right answer

quickly finding 10% of a number means to simply move the decimal to the left one place
so  10% of 300 is 30

since  5% is half of 10%
half of 30 is 15

and so  5% of 300 is 15