To do this addition mentally, you should do like this:
31 requires 9 more to become 40 and since we have a nine in our question, we would first do 31 + 9 = 40. Now you would finish it off with the remaining number which is six. 40 + 6 = 46.
Steps:
31 + 9 = 40
40 + 6 =46
Therefore, your answer would 46.
