Resistance vs Vulnerability?

[Dragon78]

If a creature is both resistance and vulnerability what do you do first....?

Do you increase the damage by 50% first then apply the resistance?

Or do you apply the resistance first then whatever is left(if any) gets the 50% increase?

[Pizza Lord]

You calculate the total damage it would take, then reduce the damage with resistance.
A creature with fire resistance 10 caught in a fireball that deals 20 fire damage would take 10 fire damage. Its partner next to it that has fire resistance 10 and fire vulnerability would take 30 fire damage reduced by 10 to 20 fire damage.

[Mysterious Stranger]

The order of operation for math puts multiplication and division before addition and subtraction.