The seller has offered him the TV for 1000 - 100 = $<<1000-100=900>>900.
The discount on the television was 900 * 20/100 = $<<900*20/100=180>>180.
The final price of the television is therefore 900 - 180 = $<<900-180=720>>720.
Martin was able to save 1000 - 720 = $<<1000-720=280>>280.
#### 280