When it comes to adding things together as quickly as possible when numbers are being tossed at you in a hurry, the first thing you want to do is to put them in order in your head in a way that you can add them together as quickly as possible. To do so you will have to arrange the numbers from biggest to smallest first, while 1 + 8 and 8 + 1 both amount to 9, there is less work involved for your brain when you are adding 1 to 8 as opposed to adding 8 to 1.
The exception to this rule is if you can put numbers together to form units of 5 or 10 which are almost always going to be easier to do first.#6499•
Chapter 7
Adding 10 to an existing number is likely one of the easiest types of addition to complete. As such, you can speed up your mental addition if you are stuck on an equation that requires you to add 7, 8, or 9, by simply adding 10 and then subtracting 1, 2, or 3. For example, 8 + 7 might require a few seconds of thought but 10 + 10 – 5 can be done simply and effectively#6490•