find a 3 digit number less than 500 in which the 3 different digits are add up to 8.

Answer 1

shouldn't be too hard to list them, if you do it systematically

116 125 134 143 152 161 170
215 224 233 242 251 260
314 ....
...
413 422 431 440

now eliminate all those with repeating digits, such as
116 224 242 332 ...etc