The grocery tally
At the checkout you add $19 + $6 by thinking '$20 + $6 = $26, then take away the extra $1' — $25. Rounding to a friendly number and adjusting is how people add fast without a calculator.
Round, add, adjust
To add a number near a ten, round it up to the ten, add, then subtract what you added. 48 + 7 → 50 + 7 = 57, minus 2 = 55.