It would be nice if we knew a bit more about your issue, but without further explanation we have to assume this is a variable of some sort..
Losing means subtracting, so..
If it's a global variable the action would be
System subtract from variable : random(100)
if it's an instance variable the action would be
Sprite subtract from variable : random(100)
this will subtract something between 0 and 100
I told my dentist I had trouble with my teeth and asked her to fix it without looking in my mouth..