Quick Addition Tip – Adding 5 to Larger Digits

For some reason I have trouble adding 5 to the larger digits (like 7, 8 and 9). I noticed at some point that the last digit of that addition is the same as if you subtracted 5 from that number. Like this:

• 7 + 5 = 12
• 7 – 5 = 2

and

• 8 + 5 = 13
• 8 – 5 = 3

and

• 9 + 5 = 14
• 9 – 5 = 4

So now when I add them, I merely subtract them and slap on a 1 at the beginning!

(Oh, yeah, and there is a good reason this happens – it has to do with 5 being half of 10. And 10 is the base of our number system. Maybe one of the other math blogs can do a proof of this using base n…?)