How to improve your calculations
The first step in improving your calculations is to learn tables, squares, cubes and fractions by heart. This helps you recognize numbers which you generally do not encounter in normal routine. By learning these numbers through tables, squares and cubes you get to know their factors and multiples which also help you in solving problems.
For multiplying numbers like 107 & 109 or 87 & 113, you can use the technique of base multiplication. To multiply 107 and 109, take the base as 100. These numbers are 7 and 9 more than the base. In order to get the answer add 9 and 7 in the numbers to the base and get the number as 116 (100 + 7 + 9). This becomes the left hand side. Take the product of the two earlier numbers i.e. 9 & 7 and get the product as 63. Now just attach this at the end of the earlier result i.e. 11663.
We can use knowledge of standard fractions to reduce calculation time. Say you want to find 91.67% of 2400. Now finding this will be really tough, if you take the actual numbers into consideration, but knowing that 91.66% is actually 11/12 of a number, you can just divide 2400 by 12 and then multiply by 11 and find the answer as 2200.
Similarly calculating 42.85% of 700 will be quite a time consuming process, but knowing that 42.85% is 3/7 of a number, you can answer this within 5 seconds that the answer will be 300.
To conclude, memorize the decimal equivalents of fractions starting from 1/2, 1/3, 2/3, 1/4, 2/4, 3/4......11/12.
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