Learn mental arithmetic

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With today's calculator, you may wonder why we need mental arithmetic. Well, just learn it for your interest or as a mental training. Let's introduce some basic calculation for mulplication. Here it goes... 0943154831463612479231473914614621146465147984459123447*2 = 1886309662927224958462947829229242292930295968918246884 You may wonder how to do this, the trick is this: the first digit is 0, 0*2 = 0, and by observe that the second digit is 9, I can know that 9*2 = 18 = 10 +8, so I can know that the first digit of answer is 0 + 1 = 1. the second digit is 9, 9*2 = 18 = 10 + 8, I take 8 but discard 10 because it already been calculated into first digit answer, now by observing the third digit, actually is the part 43154831...59123447, I know that no value will be added to the second digit answer, thus 8 + 0 = 8. Continue by following same procedures, I can write the answer within seconds from left to right, as differ from traditional calculation that have to be done from right to left.