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please provide some tips and tricks to do that mentally fast
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999 - (whatever number under 500), subtract each of the three digits from 9.

Now add one to the result, making (1000 - the number)

Now (that second result - 500) is (500 - the number).

Or simply 499 - the number (there isn't any carry) and then add 1 to the result
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i getting troubled everytime doing that mentally but with paper pen it's not so much problematic
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If I want to subtract 364 from 500, I refomulate the problem.

How much do I need to add to 364 to get to 500. I add 6, 30 and 100, so 136.
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I guess the best way is to systematically approach it by climbing up to the next hundred. Like if you’re subtracting 236 from 500 then get to 240 then to 300 then to 500. So you added 4 then 60 then 200 and indeed the answer is 264.
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