Hello and welcome @Anand_Ashar!
Automations do not currently allow you to use mathematical operators when setting fields:
To get around this you could write a custom script to use as your Automation action that would allow you to update a field on a record using whatever equations you’d like.
Let me know if I can help provide any more information and if this helped answer your question please mark it as the solution .
Thank you so much. Appreciate the help!