Answer:
26 minutes
Step-by-step explanation:
The rate of filling the tank matches the rate of discharge from the pipe. Each rate is the ratio of volume to time. Volume is jointly proportional to the square of the diameter and the height.
Volume
For some constant of proportionality k, the volume of discharge in 60 seconds from the pipe is ...
V = k·d²·h . . . . d = diameter; h = rate×time
V = k(0.64 dm)²(0.0205 dm/s × 60 s) = k·0.503808 dm³
For the tank, the height (h) is the actual height of the tank. The volume of the tank is ...
V = k(0.76 dm)²(23 dm) = k·13.2848 dm³
Proportion
Then the proportion involving (inverse) rates is ...
time/volume = (fill time)/(k·13.2848 dm³) = (1 min)/(k·0.503808 dm³)
fill time = 13.2848/0.503808 min ≈ 26.369
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Additional comments
1 dm = 100 mm = 10 cm = 0.1 m
1 dm³ = 1 liter, though we don't actually need to know that here.
We have used 1 decimeter (dm) as the length unit to keep the numbers in a reasonable range. We have worked out the rate numbers, but that isn't really necessary (see attached).
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The value of k is π/4 ≈ 0.785398. We don't need to know that because the values of k cancel when we solve the proportion.