I will admit that I do simple addition and subtraction in my head by counting, in some cases. I mean, I figure 29-5 by going '28, 27, 26, 25, 24.' It's stupid, but I never learned how not to do it. That is, I know some addition and subtraction instances instantly, but others I count. Needed more flash-card work at the right moment in second grade or something....
But I know many mental shortcuts, too. Subtracting nine from something, for example, is simple: 1 less than subtracting 10 from it.
10% is easy-peazy, as Pio was saying. Move decimal point. So 10% of 99 is 9.9. Even 10-percent increments then are easy to, just mulitply the 10% amount by whatever. 30% of 99 is 3 x 9.9. So how much is that? Shortcut: 3 X 10, -.3 = 29.7.
Like to tip 15%? Figure 10%. Then take half of the 10% amount, and add it in. Don't fuss with decimals, round up as you go. Precision not necessary here.
Need exactly 37% of 99? OK, lets get 7%. Start by getting 1%: that's done by moving the decimal point TWO places, giving .99. Multiply that by 7. As above, 7 X 1, -.07 = 6.3. Add 6.3 70 29.7 (that's 30%, figured above) = 36.
There may be better ways, I'm just sharing mine....
