Question

find the area of a thin sheeting required for making an open cistern (i.e.,without a lid), 5m long, 3m wide, 4m deep

Answer 1

Surface area of cistern = 2 ( l*w + w*d + d*l ) = 2 (5*3 + 3*4 + 4*5) = 94 
But the cistern is open, so we remove area of the lid, i.e. 3*4=12 

Answer : 94 - 12 = 82 m²

Answer 2

the area of cistern with one side open = area of 4 walls + area of base
                                                             = 2(l+b)h + lb
                                                             = 2(8)4 + 15 = 64 + 15= 79m²
ans = 79m²

regards 
PRIYANSH