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.  Usi...

Obscura: The Integral and Fractional Parts In my last post ( Foundations-Halving ), you learned that when halving an odd digit, you should treat it like the next lowest even number. For example, in 30 ÷ 2 we treat half of 3 the same as half of 2. Formally, this is accomplished using the mathematical function known as "the integral part." The integral part of a  is written with brackets, [ a ],  and identifies the unique integer a –1 <  [ a ] ≤ a . Similarly, the fractional part is denoted with curly brackets, { a }, such that { a } = a  – [ a ]. For example, the integral part of 3 ½ is 3  and the fractional part of 3 ½ is  ½ . When I say that half of three is one, mathematically I mean [3 ÷ 2] = [1 ½] = 1 . So, if it bothers you when I write  ½  ⋅  3 = 1, then just know that what I mean is [ ½  ⋅  3] = 1.

To Halve or not to Halve... That is the Question. As with the last post, let's begin with a problem. Try and mentally calculate the following division problem:  724898 / 2 .  As before, we will return to this problem at the end.  If you have an even number of items—let's say six jelly beans—and you halve them, then you divide them evenly into two groups—three jelly beans for you and three for a friend. The way we are using it here, you can't halve an odd number. Halving is dividing by two. Only even numbers are divisible by two. As martian mathematics is largely a single-digit technique, we really only need to know how to halve single-digit numbers. We will start with the easy digits: even numbers. Instead of dividing by two, I will denote halving by multiplying by a half. Halving a number with all even digits is as easy as doubling a number with all digits less than five. You can just read off the answer as quickly as you can read off the digits. Let's tr...

Doubling is ×2 Before we begin, I would like you to try your hand at the following multiplication problem (in your head): 2  × 734829.  If you would like to be able to multiply this as easily as reading of the digits, then read on.  We are all familiar with doubling. If you have  N  items in a bag and you double it, then it is like adding a second identical bag so that now we have twice as many as we did before ( 2N ).  In martian mathematics you need to know the following doubles: What about the numbers 5-9? We will get to that in a moment, but first let's take what we have out for a ride.  If we want to double a number of any size then we just read it from left-to-right one digit at a time, doubling as we go. Easy, huh? Now let's tackle the other five doubles you need to know. For the most part, martian math is a single-digit technique and we can ignore anything not in the ones' place. For this reason, when we double a number great...

Even and Odd Numbers Even numbers are the numbers of sharing jelly beans among two friends. You make sure that they are split evenly, grabbing two at a time and dropping one in each bowl. This is what it means for a number to be even. If you placed both bowls on the opposite ends of an old-fashioned scale, then they would balance. That is unless you accidentally drop an extra jelly bean in your own bowl, tipping the scales so to speak in your favor; then you have an odd number of jelly beans.  This is what it means for a number to be odd. The first five even numbers are 0, 2, 4, 6, and 8. The first five odd numbers are 1, 3, 5, 7, and 9. As martian mathematics is largely single digit in nature, these are the only ones that matter here. It is a happy coincidence that the evenness or oddness of a number—called parity—is betrayed by the ones' place value of the number in question  (I will show why this is in a later post) : 332 is even, while 241 is odd.  Kno...

Adding and Subtracting Three is as Easy as 147.  147? I told you it was weird. Hopefully you have done your homework since my last post and studied the telephone dialing pad. The first thing we need to do is rotate it 90° counter-clockwise and then move the zero to the top left next to the 3 key. Now, let's skip count in 3's starting at 0. Follow along the telephone sequence with your finger:  3, 6, 9, 1 2,  1 5,  1 8,  2 1,  2 4,  2 7,  3 0.  We are back at the beginning of the sequence, 0. Do you see what happened to the tens' place when we changed rows? It incremented. If you commit this sequence to memory, then you can easily add 3 to any number. For example, 34 + 3 = 37 because 7 comes after 4 in the telephone sequence and since it is on the same row, the tens' place value does not change. How about 28 + 3? Since the next number in the telephone sequence is 1 and it appears on the next row, we increment the tens' place ...