I can now calculate special chances in my head almost instantly. It was quite a revelation when it clicked. Maybe everyone could do it, if they spent long enough figuring out how they can do it - not everyone does maths in the same way, different methods work for different people. Here's roughly how my brain does it:
Find the nearest 5%. Did it end in a 3 or an 8? Then go up rather than down. Other rounding directions are then obvious to me, and 3 and 8 look similar. For results of 50 or below, I can divide by 5 straight away. For results above 50, if the result is a multiple of 10, either double it and divide by 10 (for odd multiples of 10) or divide by 10 and then double (for even multiples of 10). e.g. 60 => 6 => 12. Otherwise, round to the nearest even multiple of 10, divide by 10 and double, then add or subtract 1. e.g. 75 => 80 => 16 => 15, or 45 => 40 => 8 => 9. Sometimes I don't go to the nearest even multiple, if I've recently done the calculation for the odd multiple.
That may sound complex, but I can do in under 2 seconds, I don't have to think about every decision, they just happen that way automatically. The important thing to me was to round the number first before dividing or multiplying.
I now accept that "it is easy for me", but I still contend that it is not easy for everyone.
I have always had that knack.