The Brainliest Answer!
Ok, so here's what we have got. 
✨ For the pipe
r₁=d₁/2=2/2=1 m
r₂=d₂/2=1.5/2=0.75 m

✨For the cuboid [tank]
h=3m [because we only need the volume till the height of 3 meters]

Now, the volume of the cuboid [tank] is lbh= 150*100*3 meters
                                                                    =45000 m³

then now we know that speed=distance/time, where we have been given the speed. [please don't notice my grammar mistakes]

here, speed of water is 15 km/h and we are given the integers in meter, so we are going to convert the speed into the meter/second first. 
and the speed is......
=4.167 meters per second.

Now, the water flows on 4.167 meters per second. and the volume of the tank is 45000 m³. Now, we have got the speed and the volume. It was all okay till here. But from here, it the question made me think.
[I'm not sure if I'm right or wrong from ahead though I'll try]

so, to find the time, we might divide the area by the speed. [speed=distance/time, but here what we have got is the area, that's why I'm confused]

so, using the area in spite of the distance, here's what we have got...
       =10800 seconds 
       =3 hours

[I'm going to ask this question to my maths teacher today evening to make sure if I'm right or now, till that time, this was my try.]

hope it helped. 

Y'know it was an INSANE! question for me. Really, you've got a nice question. ^_^
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