50 percent off 800

50% off 800 is 400

800 minus 50 percent off is 400

The discount itself is 400: 800 × 50 ÷ 100 = 400. Taking it off 800 leaves 400.

Only approximately: 400 ÷ 0.5 = 800. The discount is rounded to the cent, so several prices give the same 400 — 800 (discount 400) and 800.01 (discount 400.01); the paid price alone cannot tell them apart.