{x:x is a positive integer less than 10 and 2^x -1 is an odd number}
Write the above set in roster form

Positive integers less than 10 = 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9

Now, applying second condition we get,

we know that 2^x is always a even number.

So when 1 is subtracted from  gives an odd number.

⇒ Roaster Form of Set A = { 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 }.