Question

A wooden box of thickness 0.5 cm is of externsl dimension 21 cm ,11 cm and 6 cm. Its inner surfaces are to be painted at the rate of ₹160 per cm square

Answer 1

The wooden box has dimensions 21cm, 11cm and 6cm. The thickness is O.5cm. So,its inner dimensions would be 21-0.5=20.5cm, 11-0.5=10.5cm and 6-0.5 cm=5.5cm.
The area of the inner surfaces = 2(lb+bh+hl)
                                  =2(20.5X10.5 + 11.5X5.5 + 5.5X20.5)
                                  =2(215.25 + 63.25 + 112.75)
                                  =2 (278.5 + 112.75)
                                  =2 (391.25)
                                  =782.5 sq. cm
1 sq. cm=Rs. 160
782.5 sq. cm= 782.5 X 160
                     = Rs. 1,25,200

Answer 2

Inner dimensions of box
Length=21-1=20
Breadth=11-1=10
Height=6-1=5
Total surface area of box=2(lb+bh+LH)
=2(20×10+10×5+20×5)
=2(200+50+100)=2(350)=700
Rate of painting=160×700=112000