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If we see the rate, it's 0.7m/sec
that means 70 cm in one second
So,in one second volume of water in pipe = π r² h
                                                         = π×(1)² × 70 = 70π
in 30 min,volume = 30×60×70π = 126000π
this volume = volume of tank
⇒π×(40)² ×H = 126000 π
⇒H =126000/1600 = 78.75 cm

6 4 6
why did you multiplied 30x60x70Π
to change 30 min into second...we have to multiply by 60