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With the widespread proliferation of computers, we can make any calculation with great ease. But it is still important that we can do these calculations in our head, chiefly to aid in estimations of approximate values in science and technology and business contexts. The important mental mathematics rounding off skills - adding numbers that are close to a multiple of 10, multiplying by powers of 5, dividing by powers of 5, dividing by 4 and 8 - are things that we should drill again and again till we can do them subconsciously, and very quickly. These exercises each have a target time, and answers are provided.
Go on, get ready to turn into a computer!