Answer:
The volume of the tank when it is filled with water =
V
1
=
(Base area)
×
Height
=
(
10
×
8
)
×
7
=
560
cu ft
The volume of the tank when it is filled with water and rocks
=
V
2
=
(Base area)
×
Height
=
(
10
×
8
)
×
8.5
=
680
cu ft
Volume of rocks
=
Volume of water displaced by the rocks
=
V
2
−
V
1
=
680
−
560
=
120
cu ft