All I was able to find about about the "Abaci" was that it is all a matter of notation. Unfortunately I was only able to find notation for fractions, not addition or multiplication. (Though there was a section about mixed numbers, eg was written by him as .)

You said you got two of them. If it isn't simply a matter of PEDMAS how did you get the answers? (If, for example, 5*6 + 5 = 35 and 7 - 5 = 2 then what you say may not actually be helpful, but give us what you've got.)

-Dan

Incidentally 4*6 = 24 is correct, unless again there is a notation issue...