Aptitude - Volume and Surface Area - Discussion

Total Surface[2(lb + lh + bh)] - (upper most surface of water)[lb]
tis concept is clear bcoz water is not touching surface of tank
but i m not getting why 2(bh + lh) + lb this step is used.. specialy (+ lb)

@kittu : [2(lb + lh + bh)] - (upper most surface of water)[lb]
2lb + 2lh + 2bh - lb
2(bh + lh) + lb

hope ya gt it..!!

This lb lh bh Means?

@Rajesh.
l means length, b means bridth and h means height.

Confusion
the part that is not wet might also contain some height so why only LB
H should be included i think so?
Plz explain

A--B
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C--D

@Rahul: it contains water upto the height 1.25m,
So we calculate the total surface up to this height,

But the above surface AB will not be wet so we will deduct AB surface area which is l*b from total surface area which also include the height.

This is a Cuboid because first we consider cistern as cuboid and second there is no diameter mention here which is possible in cuboid.

Cuboid consist total six face so total surface area is 2(lb+bh+lh).
And here lb+lb is bottom and top surface.

So we'll consider only bottom which is wet and minus the top face(AB).

Just consider the cistern as a cuboid with length l = 6 m breadth = 4 m. The total height of cistern is not given but it is given it is filled with water to height h = 1.25 m but the actual height of cistern can be anything which is not needed here.

It has bottom floor and upper ceiling, two side walls with length 6 m and 2 side walls with length 4 m.

So the areas which get wet are the bottom floor(lb), 2 side walls of length 6 m up to height 1.25 m which is 2(lh) and other 2 side walls with length(breadth)4 m which is 2(bh).

So area of wet surface = lb + 2 lh + 2 bh.

= 6*4 + 2*6*1.25 + 2*4*1.25.

= 49 sq.m.

We have 5 wet sides i.e., complete bottom portion (6mx4m = 24m2) and four walls (6mx1.25mx2 sides = 15m2 and 4mx1.25mx2 sides = 10m2).

Totally = 24+15+10 = 49m2.

2lh+2bh+lb what is this formula for I mean area of what?

[2 (lb + bh + lh) - lb].

This means that assuming the area of cuboid which has size side that we know.

So length*breadth for two surface + breadth*height for another two surface + length*height for rest two surface. Now this length*breadth is the area of a rectangle which is the outer part of the water surface not to be included.

So should be deducted.