Chrissy,
If default is 1/8 notes, dotted 1/8 followed by 1/16 is the little arrowhead (at the bottom of the keyboad) pointing to the right between the two (originally) 1/8 notes. Reverse for 1/16 followed by 1/8. This save a lit of time compared to x3/2y/2.

But! Be carefull, if you post to a site using html, all between any arrowhead pointing left to the next one pointing right will disappear in the display! The ABC hompage has an ASCII file on the rules for encoding in ABC, that tells you how to do such things. It's free too.