Think of the display as a scale, but in integers, so work in half stars. For example, if we have 5 stars, and can display half stars, our real scale is from 0 to 10. So, what we need to do is divide the original scale (0 to 100) down to give us 0 to 10. Then we do integer division by 2. The quotient of that will give us the number of complete stars to draw, and the remainder the number (0 or 1) of half stars to draw.
You may want to do some rounding first though -- otherwise, a rating of 5 stars will be essentially impossible to get (e.g., even an input score of 99 will still only give 4.5 stars).