While I normally do my Leapfrog Division posts about a year apart, I though I'd wrap up this mental division series just 1 week after the previous entry.
In this post, you'll learn to mentally divide by numbers ending in 2!
STARTING POINTS: This is the most advanced technique of all of the Leapfrog Division posts, so you should be familiar with and practice the the previous techniques. Not only does this employ the basic ideas taught in the original Leapfrog Division post, but also the subtraction from 9 idea used in Leapfrog Division II, AND both the doubling and halfway-comparison concepts from Leapfrog Division III. If you comfortable with all of these concepts, then you're ready to move on to this version.
This version also introduces a new idea to the Leapfrog Division series: The stopping rule. In the previous versions, you could stop either when you realized the numbers were going to repeat, or when you didn't need any further precision. While the same is true here, in this version, you'll also need to stop when you get a quotient of 5, and a remainder of 0. You'll understand this better, including the exceptions, as you work your way through the technique.
THE TECHNIQUE: For our first example, we'll use 17⁄32. As in the technique for dividing by numbers ending in 1, we will always start by reducing the numerator by 1, giving us 16⁄32. Similar to the method for dividing by numbers ending in 8, we're also going to compare numbers to the half of the denominator. Anytime the numerator is greater than or equal to half of the original denominator, we'll reduce it by 1.
In the case of 32, half of that is 16, so we ask if the current numerator is greater than or equal to 16. In this example, it's currently 16 (exactly equal to half!), so we reduce by 1 more, giving us a division problem of 15⁄22. As with all the Leapfrog Division techniques, we're now going to round the denominator to the nearest multiple of 10, and then divide it by 10. So, the problem becomes 15⁄3.
You shouldn't be surprised that we're going to divide this out using quotients and remainders. Starting with 15⁄2, we get:
- 15 ÷ 3 = 5 (remainder 0)
From here, you're going to use the doubling idea as taught in Leapfrog Division III, in which you double numbers, but only keep the ones (units) digit. 5 doubles to 0, because 5 × 2 = 10, and we only keep the ones digit, which is 0. Next, as in Leapfrog Division II, you're going to subtract the quotient from 9. In this example, 9 - 0 = 9, so the new quotient is now 9. Leapfrogging the remainder, 0, to the front of the quotient, we know have 09, or simply 9.
Before dividing, we need to ask whether 9 is greater than or equal to 16. It isn't, so we don't decrease the number at this stage. After that question, only then do we do the division again:
- 9 ÷ 3 = 3 (remainder 0)
- 3 ÷ 3 = 1 (remainder 0)
- 7 ÷ 3 = 2 (remainder 1)
- 15 ÷ 3 = 5 (remainder 0)
Now that you understand the steps, let's work out 19⁄22 as a second example. We start by reducing 19 by 1, which is ALWAYS the first step, giving us 18⁄22. Half of 22 is 11, and 18 is greater than 11, so we decrease it by 1 again, leaving us with 17⁄22. Rounding the denominator down and dividing by 10, our starting problem should be 17⁄2. We start there, and work through the problem this way:
- 17 ÷ 2 = 8 (remainder 1)
(8 doubles to 6, 9-6=3, 1 makes it 13, which is MORE than 11, so 13 - 1 = 12.)
- 12 ÷ 2 = 6 (remainder 0)
(6 doubles to 2, 9-2=7, 0 makes it 07, which is less than 11.)
- 7 ÷ 2 = 3 (remainder 1)
(3 doubles to 6, 9-6=3, 1 makes it 13, which is MORE than 11, so 13 - 1 = 12.)
TIPS: Yes, this has more steps than any of the other approaches taught in the Leapfrog Division series, and it's not difficult to confuse the steps of the various versions. The solution, as always, is practice, practice, practice!
You may have noticed that I referred to this as the last post in the Leapfrog Division series. Why is that? Because using the 4 different techniques I've taught, you can actually handle dividing by most numbers with just a little adjustment. How do you handle numbers ending in...
- ...9? Use Leapfrog Division.
- ...8? Use Leapfrog Division III.
- ...7? Triple dividend and divisor, so divisor ends in 1, and use Leapfrog Division II.
- ...6? Double dividend and divisor, so divisor ends in 2, and use this technique.
- ...4? Double dividend and divisor, so divisor ends in 8, and use Leapfrog Division III.
- ...3? Triple dividend and divisor, so divisor ends in 9, and use Leapfrog Division.
- ...2? Use this technique.
- ...1? Use Leapfrog Division II.