How to make it clear that a value is being changed using keynote I am using keynote to make a presentation.

Let's say that following 3 lines. And the third line is something like


We are opening a section for kids
In June use 20% discout on all kids items
On all kids items 30 days money back guarantee


Next I want this to happen.


When I hit next then a big "X" come over slowly on "20%" 
When I hit next then I want "20%" to be replaced by "40%"


How do I do that in keynote. I just purchased keynote so I am using the latest version.
Use the Inspector tool to add build-in and build-out transition to your texts and cross.  

You'll have 4 build order steps :  


cross fade in 
cross disappear
text "20%" disappear
text "40%" appear


Play with duration and 'start build' settings to obtain desired effect.
See http://www.youtube.com/watch?v=1X-5AizWIJE for a screencast ;)