What is rounding estimation, and how do you keep the estimate honest?

Round each number to something easy, compute, and track whether you rounded up or down so you know which way the true answer leans.

• 312 + 488 → 300 + 500 = ~800 (actual 800).
• 47 × 19 → 47 × 20 = 940, but you rounded 19 up by 1 (≈ one 47 too many), so the true value is a bit under 940 (actual 893).

Rounding both numbers the same direction inflates or deflates the estimate predictably; rounding in opposite directions tends to cancel and gives a tighter estimate.