Answer:
Ā about 769 cubic inches
Step-by-step explanation:
Before the stone is added, 20% of the tank volume is empty. The empty volume is ...
Ā 0.20LWH = 0.20(36 in)(12 in)² = 7.2Ć144 in³ = 1036.8 in³
The volume of the sphere is given by ...
Ā V = (4/3)Ļr³ = (4/3)Ļ(4 in)³ = 256Ļ/3 in³ ā 268.08 in³
Then the remaining empty volume after the stone is added is ...
 1036.80 in³ -268.08 in³ = 768.72 in³
The empty volume in the tank after the stone is added is about 768.72 in³.