Five is the Evenest Odd of Them All If I told you that dividing the number 74230 by 5 is about as easy as it gets, would you believe me? Clearly you can't rely on the old "five times WHAT is equal to 74,230," because nobody memorizes their times tables up to five digit numbers. So there must be a trick, right? Indeed there is. But first... Before writing this post I googled "what is dividing by five good for." Can you guess what I found? That's right. Nothing. Sure, it's handy if you need to figure out how to share a pizza amongst five friends and the like, but it doesn't have the gravitas of doubling or halving. Five is an odd number and odd numbers are appreciated for their symmetry-breaking aesthetics. Three is too few, and seven is too many. Five is just right! Well maybe it is or maybe it isn't, but it is darned easy to divide with. A number is divisible by five if it ends in zero or five. Depending on how you look at it, th
The Multiplication Workhorse Let's begin again with a problem. Can you easily multiply 484 × 5 in your head? If so, then how about 3261 × 5? By the end of this post, you will be able to easily multiply any number by five provided you have mastered doubling and halving discussed in my previous posts. Multiplication by five is the workhorse of martian mathematics. At least it is for the intermediate methods of multiplying times 3, 4, 5, 6, and 7. More on that in a later post. I think that everyone would agree that aside from multiplying by one, nothing is easier than multiplying by ten—just add a zero onto the end of the number. For example, 37 × 10 = 37 0 , 271 × 10 = 271 0 , and hell × 10 = hell 0 . Well, maybe not that last one, but you get the idea: just tack a zero onto the back of the number when multiplying by ten. Multiplying by five is almost as easy as multiplying by ten because five is just half of ten. Using the commutative property of multiplic