The rogue has the Uncanny Dodge class feature, which says this (PHB, pg. 96):
Starting at 5th level, 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.
Spiked Bone Club. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) bludgeoning damage plus 2 (1d4) piercing damage.
The Grimlock attacks the level 5 rogue and hits, and the DM rolls damage. The first d4 gives a 2, giving a total of 5 bludgeoning damage, and the other d4 gives a 3, giving a total of 3 piercing damage. The grand total is 8 damage.
- round each individual total down, which in this example is (5 / 2) + (3 / 2) = 2 + 1 = 3?
- round down the grand total, which in this example is (5 + 3) / 2 = 8 / 2 = 4?