If an enemy hits a Rogue with an attack, the Rogue uses Uncanny Dodge as a reaction, and at the same time a Fighter uses their reaction to use Interception to lessen the damage the Rogue takes: which feature takes effect first?
When a creature you can see hits a target, other than you, within 5 feet of you with an attack, you can use your reaction to reduce the damage the target takes by 1d10 + your proficiency bonus (to a minimum of 0 damage). You must be wielding a shield or a simple or martial weapon to use this reaction.
When an attacker that you can see hits you with an attack, you can use your reaction to halve the attack's damage against you.
So let's say the attack deals 30 damage, and both Uncanny Dodge and Interception takes place. The Fighter rolled an 8 for the Interception. Does the total damage taken becomes:
a. (30 - 8)/2 = 11, or
b. (30/2) - 8 = 7 ?