Would that work?

Well - you will notice that, for all numbers from 10 to 19, the answer is always 9, for all numbers from 20 to 29, the answwer is always 18 . . . and so on 27,36.

Using the unfamiliar symbols disguises this.

If you did it to base 8, the answers would be [7],[16[, [25] for [10] to [17],[20] to 27], [30] to [37]. I think my sums are correct. I leave it to the student to work out the results for base 16.