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The volume that enters the tank per minute is 12 ft³, and water rises by 6 in.
Convert: 6 in = 1/2 ft  

We now have:
Δh = 1/2 ft
ΔV = 12 ft³

Since r is constant, 
ΔV = BΔh = πr² Δh

12 ft³ = πr² 1/2 ft

24/π ft² = r²

r = √(24/π) ft