a paper box without a lid is 25 cm long. 16 cm wide and 5 cm deep . how many square centimetres of paper have been used to make the box .

visualize the box cut open and laid out flat on a table.
What do you see ?
Calculate the area of each of the 5 rectangles and add them up

Given,
Length of SQ. box= 25 cm
Width= 16 cm
Depth= 5 cm
Area of rectangle 1= 25×5
= 125 cm2
Area of rectangle 2= 25×5
=125 cm2
Area of rectangle 3= 16×5
=80 cm2
Area of rectangle 4= 16×5
=80 cm2
Area of rectangle 5= 16×25
=400 cm2
Total area of paper box= 400+80+80+125+125 = 810 cm2

(Note: Visualize a paper box consisting of 5 rectangles when laid out flat. Draw this and number the rectangles calculating the area of each separately using the formula 'Rectangle= length×breadth'. Add all the answers up and voila! The total area.)