A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
You can do this on the master, if the numbers are sequential and on every slide. Use View>Master>Slide Master to open it. You'll sere is already a page number field there. Make the field narrower and change the page number to be flush left. Then, just to it's left create a text box and set the point size and color to match the page number. Type in 24., then move the text box so it plus the page number look like 24.1.
If the numbers are not sequential or not on every slide, then just create a small text box and type in the number for that slide. If you copy and paste this onto other slides, the position will remain constant.