okay so the first thing to do is list the possible combinations for the sum of 11. And you end up with 3 +8, 4+7 and 2+9. I excluded 1+10 because it does not fulfill the first requirement (the number has to be double digit since it is between 20 and 50). I also excluded 5+6 because together they form 56 which does not fulfill the first requirement (between 20 and 50). Okay so the possible answers are 38, 47 and 29. So to eliminate the other 2 possible answers, use the 6 times table. You will come to realise that only 38 fulfills the second requirement. (since 6 x 6 =36, so remainder for 38 will be 2)
You can try it out on the other 2 options and you will realise that they will give you remainders of 5 instead so 38 is the answer. Hope this helps you!