One way to calculate this is finding what the percentage of 160 is out of 200.
160/200= 0.8
You then have to subtract this number from 1 (since you're asking for a markdown, not up) and get 0.2.
0.2 x 100 = 20%
The markdown is therefore 20%
Alternatively, you could subtract 160 from 200 to get 40, the amount it was marked down by. Then find the percentage of 40 out of 200.
40/200 = 0.2
0.2 x 100 = 20%
