Can you do math in Klingon?

Pia had been doing addition and subtraction for a couple of weeks now, and she's definitely getting better and better.  So much so that she's been answering pretty much all the questions correctly.

But when we moved to the next level - adding numbers with two or more columns (for example, 35 + 48), she faltered very very badly.

We could not figure out why, until we realized that she still hadn't memorized writing out numbers beyond 1 to 12.

So although she could add the numbers column by column and she could enunciate the correct answer, she didn't know how to write out the answer.

For example, she could say 'eight plus eight equals sixteen', she didn't know by heart how to write '16'.  She would write something like 8 + 8 = 65.