50 percent off 100

50% off 100 is 50

100 minus 50 percent off is 50

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

Only approximately: 50 ÷ 0.5 = 100. The discount is rounded to the cent, so several prices give the same 50 — 100 (discount 50) and 100.01 (discount 50.01); the paid price alone cannot tell them apart.