Singha has made a mistake in step 3
The actual diameter is 13.2 ft.
Explanation:
Volume of cone = 88π cubic ft.
height (h) = 6ft
We know that,
Volume = (1/3) (π x r²x h)
88π = (1/3) (π x r² x 6)
88π (3) = π x r² x 6
88π(3) /6 = π x r²
44π = π x r²
44π/π = r²
r² = 44
r = 6.6ft
diameter = 2(6.6) = 13.2ft