Order Confirmation Email - performing multiplication

Hello, 

I would like to edit the order confirmation email. At the bottom, I want to add a line which refers to the tax amount paid by the customer. I want the line to be called "TVA 20%" and it needs to show the amount of TVA which would be the result of "0.2 x order price". I don’t know how to add a calculation in the HTLM code. 
I tried to do something like that:
 

Sous-total : {{ event.extra.subtotal_price }}

TVA 20% (incluse) :  {{ 0.2*event.extra.subtotal_price }}

Livraison : {{ event.extra.shipping_lines.0.price }}

Total : {{ event|lookup:'$value' }}

Obviously, it’s not correct. Can you help me fixing the code?

Kind regards,