"Racetrack Math"

The example that you give is the exact reason to do math the way I was taught how to do it. If I were to add those two numbers I would do it this way. 25 + 40 = 65 65 + 6 = 71 The whole point is to break down into small chunks that are easy to do. Anyone can do 25 + 40 in their head and anyone can do 65 + 6 in their head. The time I use it the most is like the examples I gave which is percentages off in a store. If something is $20 and 30% off you go WHOA I can't do that. BUT, 10% is just moving a decimal to the left one spot ALWAYS. $20 * 10% is $2.0 Since it is 30 percent we do it three times 20 * 10% = 2 20 * 10% = 2 20 * 10% = 2 2+2+2=6 and we subtract that from 20 20 - 6 = 14. It is to make math simple. Instead of learning multiplication tables you learn 1,2,5,10. That's it. What about 9 you say. There's a trick for that. Check THIS out. ANYTHING multiplied by nine... is that number multiplied by 10 minus the amount being multiplied. Sounds like a bunch of gibberish I know; but watch. 14 * 9 Can you do that in your head? I can't. 14 * 10 Can you do that... of course you move the decimal one place to the right 14.0 * 10 = 140 Now you wanted to multiply by NINE so you subtract how many you were multiplying by (14) Again that is too hard 140 - 14 I can't do it So I break it down 140 - 10 = 130 - 4 = 126. 14 * 9 = 126. The 'hard' part is keeping track of the current number in your head. Once you start thinking that way it is second nature. Of course the larger the numbers the more steps but it's always the same. 25 * 17. HOLY CRAP! Where's my calculator.? Just break it down. 25*10=250 25*7 = (25 * 5) + (25 * 2) = 175 now add the two numbers 250 + 175 = HOLY CRAP break it down 250 + 100 = 350 350 + 75 = HOLY CRAP break it down 350 + 50 = 400 400 + 25 = 425 25 * 17 = 425 All the calculations you did are calculations you or any 3rd grader can do Your brain learns to keep track of where you are in the calculation and when that happens you do these little math problems in your head so fast that you can do a two-digit times two-digit number faster than someone with a calculator. Just try something simple like go to the store and find something 25% off. $17.99 SALE: 25% off! Round the ridiculous $17.99 to $18 Now we do the math "backwards" because we want to know the price not the amount off. So we do $18 * (100% - 25% = 75%) $18 * 75% Now break it up (70% and 5%) $18 * 10% (we need to do that SEVEN times) What I do is multiples of 2 (a number that can be used [1,2,5,10]) and say 18 * 2 = 36 36 + 36 = 72 + 36 = 108 So we did 2 times and two times and two times = six times we need to add 18 more 108 + 18 = 126 (again I am shortcutting here. you can break down the 10 and the 8) now 5% is exactly half of 10% (duh right! this is how easy it is) so 18 * 10% = 18. Divide by 2 is 9. 5% is 9 126 + 9 = 135 Move our decimal point back 13.5 $13.50 WOOHOO it's $13.50. When I talk it out loud I would say 18 times 75 18 and 18 is 36 36 and 36 is 72 72 and 36 is 108 108 and 18 is 126 126 and 9 is 135 Honey, the price is $13.50. What I did was took 70 first and created a shortcut I found 60 first 18 and 18 is 36 (that is 20%) 36 and 36 is 72 (that is 40%) 36 and 72 is 108 (the above 20% and 40% is 60%) 108 and 18 is 126 (that gets me to 70%) and 9 (I only need 5% so half 18) is $135 Move the decimal over $13.5 I found what was easiest for me and did it over and over again. Traditional math OMG... would be 18 .75 --- 18 05 -- 90 18 70 -- 1260 1260 + 90 = 1350 Move 2 decimal points 13.50 Waaay to hard to do even for me. But in less than 5 seconds I can do it IN MY HEAD with racetrack math. It's called the associative property of addition and subtraction. It's covered in like 3rd grade for a week and then completely forgotten about. My sister was HORRIBLE at math and just using the '9 trick' helped her SO much 174 * 9 = HOLY CRAP 174 * 10 "Oh that is 1740" minus 174 = HOLY CRAP (then use the associative property to break up the 100/40/30/4 (1640 - 1600 - 1570 - 1566) Just make it easy you have 1740 - 0174 ---- Take 100 off first making 1640. Take off 40 making 1600. Take 30 off making 1570. (40+30=70) Take off the remaining 4 to get 1566.

Lost me there. I still am not good at math. I am good at estimating and now what a cup looks like, and when I do math I round it out to the nearest figure that would not do me any good if you want me to build something. Your sister sounds like she does what I sort of doing it. I think that is rounding it out. What I do is instead of saying somethiong like 14 x 5, I translated that to 7 x 10 and that makes it easy.

Yeap again it comes down to getting numbers to 1,2,5,10. It doesn't have to be done in the way I do it but rather to get numbers easy enough to do in your head and the example you just gave wasn't rounding but rather getting to 10 in your own way. 5 is always double (to get to 10). Zelo

Yes 11 is addition and 9 is subtraction. Two shortcuts that makes peoples jaw drop when you can calculate big numbers like that. All you are doing is basic math. Thanks Zelo

Basically the same thing. Breaking it down to come up with the answer. There are no hard set rules on how you do it but rather what is easier for the individual. Just makes sense that way. Thanks Zelo