What is front-end estimation?

Add or multiply using only the leading digits first for a quick baseline, then refine with the next digits if needed.

368 + 421 + 195 → front ends: 300 + 400 + 100 = 800 (baseline) → then the tens/units (68 + 21 + 95 ≈ 180) → ≈ 980 (actual 984).

It gives an instant order-of-magnitude answer from the most significant digits, which you can sharpen by folding in the smaller places. Great for sanity-checking totals at a glance.