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If the Pokemon with Solid Rock is hit by an attack that would normally be x2 effective, it would be x1.33 effective with Solid Rock. If a Pokemon is hit by an attack that would normally be x4 effective, it would be x2.66 effective with Solid Rock.
To clarify.
Example o1. x2 effective hit
Attacking Pokemon: Heracross
Defending Pokemon: Rhyperior w/ Solid Rock
Attacking Move: Close Combat
Base Power: 120
After STAB: 180
Before Solid Rock (supereffective x2 hit): 360
After Solid Rock (supereffective x2 hit): 240 (180 x 1.33 = 240)
Total Power: 240
Example o2.
Attacking Pokemon: Starmie
Defending Pokemon: Rhyperior w/ Solid Rock
Attacking Move: Hydro Pump
Base Power: 120
After STAB: 180
Before Solid Rock (supereffective x4 hit): 720 (180 x 4= 720)
After Solid Rock (supereffective x4 hit): 480 (180 x 2.66 = 480) .ceiling
Total Power: 480
So basically...
x2 effective with Solid Rock = x1.33
x4 effective with Solid Rock = x2.66
It's 2/3.
... and Serebii isn't always right. It was wrong about the whole Flame Orb vs. Guts thing. Who knows what else,
In the AbilityDex for Solid Rock, it says, for the In-Depth Detail thingy, that Super-Effective moves are halved. Not 1/2, but halved (same thing). I am, however, now confused with what it really is because of the different numbers stated...