Two quantities multiply to a constant (speed × time, price × quantity). If one rises by a fraction 1/x, how much does the other fall?

It falls by 1/(x+1) — a 1/x increase in one factor forces exactly a 1/(x+1) decrease in the other.

Multiplying a factor by (x+1)/x forces the other by x/(x+1) = 1 − 1/(x+1):

• Speed 4 → 6 km/h is +2/4 = +1/2 (x = 2), so time drops by 1/3. A 90-min trip → 90 − 30 = 60 min.

Check: 4 × 90 = 360 = 6 × 60. ✓ Great for "if I go 25% faster…" (+1/4 → −1/5) style questions.