Question 1128762
5 cm from the top means that the height of the water in the tank is 30 - 5 = 25.
So we now have: L=50; B=20; H=25 
(note: "breadth" we would call "width" but I'll continue to use breadth because that's what you used)
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a) At a level of 5 cm from the top the tank holds (volume): 50*20*25 = 25000 cm^3 of water = 250 m^3
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Question b can be answered in cm^2 but not in cm^3:
b) One L*B (the bottom, it doesn't have a top) = 50*20 = 1000 cm^2
Two sides: 20*30*2 = 1200 cm^2
Front and back: 30*50*2 = 3000 cm^2
Total: 1000+1200+3000 = 5200 cm^2