How to make 30% chance spawning unit

Scripting Help
1

how to spawn unit BUT ITS 30% chance that will be spawned beacuse rarest unit

2

Trigger : Every second ( If you want to when if it hits an second etc )
Increase timer by +1
If timer = 100
then do:
If random number between 1 and ( Your number u need to calculate etc ) == 1
then do:
Create ( Your unit ) for AI ( depends on the ai you want to it be ), at the position you want etc.