A square steel is 4 meters long. Its cross section is a square with a side length of 5 cm. It is known that the weight of steel per cubic decimeter is 7.8 kg. How many kg does this square steel weigh

4 M = 40 decimeters 5 cm = 0.5 decimeters
Volume of square steel = 0.5 × 0.5 × 40 = 10 (cubic decimeter)
Steel weight = 7.8 × 10 = 78 (kg)

A section of square steel is 5 meters long and its cross-sectional area is a square with a side length of 3 cm. What is the weight of this section of square steel (1 cubic decimeter steel weighs 7.8 kg)

Li Hao,
The length is:
5 meters = 50 decimeters
Side length:
3cm = 0.3dm
Volume:
50 × 0.3 × 0.3 = 4.5 (cubic decimeter)
Weight:
4.5 × 7.8 = 35.1 (kg)

Volume of square steel = 100 decimeter x 1 decimeter x 1 decimeter = 100 cubic decimeter, weight = 100x7.8 = 780kg