[Q] How can I remember the total stopping distances?
(These figures are for dry conditions)

[A] If you start at 20 mph, which is x 2 = 40 Feet, then 30 is x 2.5 and 40 is x 3 as follwows:

This only works for measurement in feet, but you can always convert back to metres.  To convert to meters you can estimate this by dividing the distance in feet by 3.