If the diameter is 16 feet, then the radius for the tank will have to be half that,
now, that's how many ft³ of water the tank can hold, since she is filling it up at a rate of 6 ft³ per minute, namely in 1 minute 6 ft³ have gone in, then it'll be
768π/6 how many minutes it'll take.
namely, how many times does 6 go into 768π? well, 768π/6.