* (asterisk) on 8-key (RIGHT MIDDLE REACH)
+ and = on key next to backspace key (RIGHT PINKY REACH)
- and _ on key to left of +/= key (RIGHT PINKY REACH)
You already know the / (which is used for division).
This makes it possible for us to type things like:
4 + 4 = 8 (addition) 5 * 4 = 20 (multiplication) 24 / 6 = 4 (division) 6 - 3 = 3 (subtraction)