As Intruder said, logarithmic.
Log5 + log15 = log (5 x 15)
Or in CR-2 language:
5 on the outer ring "added" to 15 on the inner ring, then go around to the end of the ring (10) to get 5 x 15.
Log 15 - log 5 = log (15 / 5) using a similar idea.
You can do square roots too, but I have forgotten how...
O8.