SIL supports the standard operators (+, -, *, /, %), however, those operators can be used on more than just numbers.
Example 1 - Basic numbers
number surcharge = 2.50;
number rebate = 1.00;
number discount = 10;
number total = 10;
total = total + surcharge;
total = total - rebate;
total = total - (total * (discount/100));
return total;