Yes you are right, but I'll walk through this to explain. To start, a Water move against Rhyperior does 4x normal damage.

Holding a Passho Berry halves the damage from Water attacks.

Intense sunlight also halves the damage from Water attacks.

Solid Rock reduces damage from super-effective attacks by a quarter. (Water would still be classed as super effective even if the other effects mean the move isn't actually effective any more.)

Half is the same as multiplying by 0.5, and taking a quarter off something is the same as multiplying by 0.75.

So you end up with 4 x 0.5 x 0.5 x 0.75 = **0.75**, or 75% of a normal-effectiveness move. It doesn't matter what order they activate in, because A x B is the same as B x A.