To understand why the rule works we look at its individual component parts – it is best to tackle
this by applying the principal of finding something similar which we can cope with, and then
working out the difference between this and the case in hand.
Applying this method, we multiply first the numbers ignoring the decimal point, then
examine the effect of having the decimal points as originally given.
(Remember that a number without a decimal point is the same as that number with a decimal
point to its right, e.g. 139 = 139.0)