in TI-nspire CAS, you can set the document settings to either auto, exact, or approximate. If it's set on either exact or auto, when you enter 1.5, it should give you 3/2 automatically. However, if it is set on approximate, or if for some reason auto is not doing it for you, then you can enter the following:
*to change the document settings, click ctrl-tools>file>document settings
I hope this helps.