Estimation

Close one eye, extend your thumb to align with a distant object, then switch eyes. The object appears to shift. Multiply that apparent shift by 10 to get the actual distance. This works because the ratio between your eyes and your arm length is roughly 1:10.

There's a quick way to estimate square roots. Find the nearest perfect square, take its root, then add the difference divided by twice that root. For √17: √16 = 4, plus (17−16)÷(4×2) = 0.125, giving 4.125. The actual value is 4.123—almost exact.

The "Rule of 72" lets you quickly estimate compound interest. Divide 72 by the interest rate to find how many years it takes to double your money. At 6% annual return, 72 ÷ 6 ≈ 12 years to double. The actual result is 2.012×—remarkably accurate.