Mathematics "Estimate and Mental Arithmetic" Training : Tomoyuki Horiguchi

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Mathematics
Up until I was a student, I was not supposed to make a mistake. Even a slight mistake would result in 0 points. Now that I am an adult, I have more numbers around me than I did when I was a student. In my life and work, there are many situations where I need to do some kind of "calculation", but there are only a limited number of situations where I need to give a 100% accurate answer. In many situations, all I have to do is to do a rough calculation and grasp the whole picture.
This book touches on actual problems while understanding rough calculations and the tips and concepts of mental calculation that come with it. This book gives you the ability to quickly grasp the meaning of all the numbers that people in society come to know everyday, including point calculations for daily shopping, cost ratios of own products, and trillions of values that are the size of national budget.