To all those answering 820
Remember relational algebra operator select is also distinct
By principal of mutual inclusion exclusion
For each A>10 (11-20)
there are 20 distinct B(1-20)
So current total = 200 (20*10)
For each B = 18
There are 15 distinct A
current total = 200 + 15 = 215
Now excluding the tuples which we have counted twice i.e B=18 and A>10
There are 10 such tuples
So current total = 200 +15 -10
Final Answer should be 205 according to me
Sadly I made a silly mistake and entered 210 :)