Let Illustrator Calculate It for You

You can perform simple math in the Transform palette or the Control palette, both found under the Window menu. For example, if you need to make an object one-third of its width, just click after the current size in the Width field and type /3. Use / for divided by, * for multiplied by and + or - for, well, you know. Then press Return (PC: Enter) and Illustrator will do the calculation and transform accordingly.