Let's say the tv was 100$ . Reduced by 20% means the tv is now 100 - 20 = 80% of the original price.
0.8(100) = 80$
Reduced again; by 10% means that it is now 100-10 = 90% of the 80$
0.9(80) = 72$
sale price ÷ original price
72$ / 100$ = 0.72 which is 72% of the original price. Which is a mark down of:
100 - 72 = 28%
The new price is 28% lower than the original price.