text: how can you read one's mind using mathematical problems.? how can you read one's mind using mathematical problem  for example, select a two-number, add it, and subtract it from your original number.example: if the number you chose is 23,add the number together 2+3=5, then subtract it from the original number 23-5 will give me the number you are thinking of. hi,   for the example that you gave, the number that you choose will have two digits, a and b.   so the two digit number will be 10a +b. for example 23 = 10*2 + 3.  then if you add the two digits together  (a+b)   and then subtract them from the original number  10a + b - (a+b) = 10a + b - a - b = 9a  as you may remember, for any number times 9, if you add the digits together, you get nine.   9*1 = 9 9*2 = 18 = 1 + 8 = 9.  9*3 = 27 = 2 + 7 = 9  et cetera.  your answer will always be 9. it's a trick.   hope that helps,  matt