10 can be considered as a common factor between 200 and 600 where:
200 = 10*20
600 = 10*60
Therefore, multiplying the 10 by a 2-digit number between 20 and 60 would give a product that is greater than 200 and less than 600.
Examples:
10*21 = 210
10*45 = 450
10*53 = 530