Here is a very handy guide from Mazars for the 2012 tax year. Use the table as on page 4 of that pdf.
Using your figures as above, the calc would be like this :
Cost of vehicle is R100,000 so the fixed cost would be -
38,726 (as per table) / 20,000km = R1.9363 per km
Now add the fuel and maintenance components (R0.68 and R0.292 as per table) gives a total deduction of R2.9083 per kilometer
10,000km (business km actually travelled) * R2,9083 = R29,083 would be the total deduction allowed (limited to the amount of the travel allowance actually received)
This is a very simple example with one vehicle used for the full year using the 2012 tax year amounts.