How to calculate 5% tax of 50% percent from price total?

Hey Guys,
How i can calculate 5% tax of 50% percent from price total? We have here in my country a new tax decree of how we can calculate tax. I have tax included prices and how i can say samba to calculate only from 50% of the sum total 5% tax and the other 50% showing on ticket like exempt?
Thanks for answers…

Well 2.5% of total would be 5% of 50%…