Use of applied mathematics during hiring process for developers

Please solve a task. We have 5 similar coins, 3 of those are real and weight the same, while 2 are fake. 1 fake coin is lighter and the other is heavier than the real ones. How many weighting operations we will need to find at least one real coin?

  1. 3
  2. -
  3. 2
  4. 2 - but it will fail if both fake coins weight the same as 2 good ones
  5. 3 worst case 2 best case
  6. 3
  7. At most 2
  8. 3
  9. 1 or 2