Explanation:
8.
Let the number is x. A number minus 7 is (x-7). It is doubled to get 86. So,

The number can be 16.27 or -2.27.
9.
The diameter of the cylindrical tank, d = 20 ft
Radius, r = 10 ft
It is filled with oil to a depth of h feet.
The volume of a cylindrical shaped object is given by :

Put the given values,

So, the volume of the tank is equal to
.