5, 25, 50 'Trick'

The key to this trick is remembering that all numbers can be expressed in countless ways. When multiplying by 5, (you should know your multiplication tables, but sometimes we multiply by large numbers or numbers with decimal places), think of 5 as 10*(1/2).

For example if multiplying 5 by 365.24, you can divide 365.24 by 2 then multiply by ten by simply moving the decimal place one to the right, or by moving the decimal place first then dividing by 2. This will yield 1826.2

So, to re-express 50 to make multiplying easier, think of it as 100*(1/2). Move the decimal place right two and divide by two, or take half the number you’re multiplying by then multiply by 100. For example, multiplying 50 by 365.24 will yield 18, 262.

Now the number 25 can be re-expressed as 100*(1/4). Multiplying 25 by 365.25 will then yield 9131, which is half of 18, 262.

Multiplying a Fraction by the Numerator

When you have a fraction like (19/20) and multiply it by its denominator (19). The answer is the denominator subtracted from the numerator (usually a negative number) added to the numerator. The numerator of the remainder is difference between the two squared and the denominator is the denominator from the original fraction.
E.x.
(19/20) * 19 = 18(1/20)
19-20 = -1 + 19 = 18
then 19-20=-1, -1^2 = 1, over 20.