What is the percent markdown of $200 and $160

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%

Y=-2x-6 is the answer