In order to add two numbers less than 100, add the tens first, instead of the ones. For example, to add 29 to 45, you do (29 + 40) + 5 = 69 + 5 = 74. As an addition, you should not say, either verbally or mentally, such as 29 and 40 are 69, 69 and 5 are 74. Instead, just say 69, 74.

Try the following yourself. Please close up the answers.

23 + 48 = 63, 71

47 + 72 = 117, 119

79 + 69 = 139, 148

After some practices, the first reading should be left out. Thus the answer is gathered at once.

The only caveat to this technique is that, the calculation speed is proportional to the frequency of practices made. Without enough practices, you may not master this technique.

You can check out the book

ReplyDeleteSecrets of Mental Math: The Mathemagician's Guide to Lightning Calculation and Amazing Math Tricksby Arthur Benjamin ( http://preview.tinyurl.com/2fbun84 ). This book is full of such tricks.