A pipe of radius 0.0100 m carries water at 2.33 m/s. How much
time does it take to fill up a
1.00 m^3 fish tank?

The pipe has a cross-sectional area of

π (0.0100 m)² = 0.000100π m² ≈ 0.000314 m²

Each second, one such cross-section of water travels a distance of

(2.33 m/s) • (1 s) = 2.33 m

so that, per second, the pipe carries

(2.33 m) • (0.000100π m²) = 0.000233π m³ ≈ 0.000732 m³

of water into the tank each second. Put another way, 2.33 m of the pipe holds around 0.000732 m³ of water.

Divide the total volume of the tank by this rate:

(1.00 m³) / (0.000233π m³/s) ≈ 1366.14 s ≈ 1370 s

or about 22.8 min, or about 0.379 hr, etc.