"The wise traveler does not force a path through the thicket. A small detour to the open road will carry you farther, faster."

The Art of Rounding

When a number is awkward to subtract, round it up to something easy. Subtract that instead, then add back the small difference.

132 − 87 = ?
Step 1: Round 87 up to 90
Step 2: Subtract the round number: 132 − 90 = 42
Step 3: We subtracted 3 too many (90 − 87 = 3)
Step 4: Add back: 42 + 3 = 45
253 − 78 = ?
Round 78 → 80
253 − 80 = 173, add back 2 → 175

Now you try.

145 − 68 =
231 − 89 =
164 − 47 =
312 − 196 =
Insight

Sometimes the fastest path isn't the straight one. Round up, subtract easy, add back the small difference. You have now completed the Mental Math Koans.

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