Not a great deal is known about Roman mathematics because they were not terribly interested in it.
Doing arithmetic probably played a much smaller role in the everyday lives of Romans than it does in ours.
The Romans had signs for 1, 10 and 100, etc., but they did not have a place value system, so a sign had the same value no matter where it occurred.
This made complex arithmetical operations very cumbersome.
We do know they used the abacus or counting table.
This would not necessarily have resembled an Asiatic abacus, but could simply have been a table or smooth slab with the surface ruled into columns headed by values: units, tens, hundreds, etc.
The counters had different values depending on which column they were in.
The Romans might also have used a system of successive doubling to do multiplication.
For example, to multiply 9 by 7, set up two columns, one headed by 9 and one headed by 1. Double 9 to get 18, double 18 to get 36.
In the right-hand column, double Ito get 2 and 2 to get 4.
Using the right-hand numbers that total 7, add the corresponding left-hand numbers.
They total 63, or 9 times 7.
This system was used by the ancient Egyptians and was also used in Russia into the nineteenth century.