Constant Difference

Add the same amount to both numbers and the answer never changes. 83 - 47 is fiddly. Add 3 to each and it becomes 86 - 50, which is just 36.

Why nudge by 3? It turns 47 into a clean multiple of 10, and subtracting from a round number is effortless. The gap between the two numbers stayed exactly the same, so the easy version gives the same answer as the hard one.

This works because subtraction measures the gap between two numbers. Slide both numbers up or down by the same step and the gap is untouched, like moving a ruler without stretching it.

Once that clicks, you will start nudging numbers on instinct, steering toward whichever pair is easiest, so the subtraction barely feels like work.