Pat was a 19-year-old attending a Special School for the Intellectually Disabled in Indonesia. She was interviewed by the first author regarding her mental calculation strategies when solving 1- and 2-digit addition and subtraction problems. Results indicate that she was able to see ten as a unit composed of ten ones and was facile in using standard written algorithms: addition with or without carrying and subtraction with or without borrowing. Her mental calculation strategies were influenced by the taught standard written algorithms. These algorithms seem to be counter-productive. However, with appropriate supports, she might have a potential to be an accurate and flexible mental calculator.