Maths Tricks

Maths Tricks

Multiply Up to 20X20 In Your Head In just FIVE minutes you should learn to quickly multiply up to 20x20 in your head.  With this trick, you will be able to multiply any two numbers from 11 to 19 in your head quickly, without the use of a calculator. I will assume that you know your multiplication table reasonably well up to 10x10. Try this: Take 15 x 13 for an example. Always place the larger number of the two on top in your mind. Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need. First add 15 + 3 = 18 Add a zero behind it (multiply by 10) to get 180. Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15) Add 180 + 15 = 195. That is It! Wasn't that easy? Practice it on paper first!

The 11 Rule You likely all know the 10 rule (to multiply by 10, just add a 0 behind the number) but do you know the 11 rule? It is as easy! You should be able to do this one in your head for any two digit number. Practice it on paper first! To multiply any two digit number by 11: For this example we will use 54. Separate the two digits in your mind (5__4). Notice the hole between them! Add the 5 and the 4 together (5+4=9) Put the resulting 9 in the whole 594. That's it! 11 x 54=594 The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5+7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627 Practice it on paper first!

Finger Math: 9X Rule To multiply by 9, try this: (1) Spread your two hands out and place them on a desk or table in front of you. (2) To multiply by 3, fold down the 3rd finger from the left. To multiply by 4, it would be the 4th finger and so on. (3) The answer is 27 ... READ it from the two fingers on the left of the folded down finger and the 7 fingers on the right of it. This works for anything up to 9x10!

Square a 2 Digit Number Ending in 5 For this example we will use 25 Take the "tens" part of the number (the 2 and add 1)=3 Multiply the original "tens" part of the number by the new number (2x3) Take the result (2x3=6) and put 25 behind it. Result the answer 625. Try a few more 75 squared ... = 7x8=56 ... put 25 behind it is 5625. 55 squared = 5x6=30 ... put 25 behind it ... is 3025. Another easy one! Practice it on paper first!

Square 2 Digit Number: UP-DOWN Method Square a 2 Digit Number, for this example 37: Look for the nearest 10 boundary In this case up 3 from 37 to 40. Since you went UP 3 to 40 go DOWN 3 from 37 to 34. Now mentally multiply 34x40 The way I do it is 34x10=340; Double it mentally to 680 Double it again mentally to 1360 This 1360 is the FIRST interim answer. 37 is "3" away from the 10 boundary 40. Square this "3" distance from 10 boundary. 3x3=9 which is the SECOND interim answer. Add the two interim answers to get the final answer. Answer: 1360 + 9 = 1369 With practice this can easily be done in your head.

Multiply By 4 To quickly multiply by four, double the number and then double it again.  Often this can be done in your head.

Multiply By 5 To quickly multiply by 5, divide the number in two and then multiply it by 10.  Often this can be done quickly in your head.

The 11 Rule Expanded You can directly write down the answer to any number multiplied by 11. Take for example the number 51236 X 11. First, write down the number with a zero in front of it. 051236 The zero is necessary so that the rules are simpler. Draw a line under the number. Bear with me on this one. It is simple if you work through it slowly. To do this, all you have to do this is "Add the neighbor". Look at the 6 in the "units" position of the number. Since there is no number to the right of it, you can't add to its "neighbor" so just write down 6 below the 6 in the units col. For the "tens" place, add the 3 to the its "neighbor" (the 6). Write the answer: 9 below the 3. For the "hundreds" place, add the 2 to the its "neighbor" (the 3). Write the answer: 5 below the 2. For the "thousands" place, add the 1 to the its "neighbor" (the 2). Write the answer: 3 below the 1. For the "ten-thousands" place, add the 5 to the its "neighbor" (the 1). Write the answer: 6 below the 5. For the "hundred-thousands" place, add the 0 to the its "neighbor" (the 5). Write the answer: 5 below the 0. That's it ... 11 X 051236 = 563596 Practice it on paper first!

Subtracting from 1,000 To subtract a large number from 1,000 you can use this basic rule: subtract all but the last number from 9, then subtract the last number from 10: 1000 -648 step1: subtract 6 from 9 = 3 step2: subtract 4 from 9 = 5 step3: subtract 8 from 10 = 2 answer: 352

Assorted Multiplication Rules Multiply by 5: Multiply by 10 and divide by 2. Multiply by 6: Sometimes multiplying by 3 and then 2 is easy. Multiply by 9: Multiply by 10 and subtract the original number. Multiply by 12: Multiply by 10 and add twice the original number. Multiply by 13: Multiply by 3 and add 10 times original number. Multiply by 14: Multiply by 7 and then multiply by 2 Multiply by 15: Multiply by 10 and add 5 times the original number, as above. Multiply by 16: You can double four times, if you want to. Or you can multiply by 8 and then by 2. Multiply by 17: Multiply by 7 and add 10 times original number. Multiply by 18: Multiply by 20 and subtract twice the original number (which is obvious from the first step). Multiply by 19: Multiply by 20 and subtract the original number. Multiply by 24: Multiply by 8 and then multiply by 3. Multiply by 27: Multiply by 30 and subtract 3 times the original number   (which is obvious from the first step). Multiply by 45: Multiply by 50 and subtract 5 times the original number   (which is obvious from the first step). Multiply by 90: Multiply by 9 (as above) and put a zero on the right. Multiply by 98: Multiply by 100 and subtract twice the original number. Multiply by 99: Multiply by 100 and subtract the original number.