Concrete curb

The concrete curb has a width of 20 cm at the bottom and 15 cm at the top. It is 20cm tall. Its length is 80 cm. The density of concrete is p=2100kg/m³.
How many can Mr. Havlík load on a trailer for a car if the trailer has a load capacity of 400 kg?

Correct answer:

n =  6

Step-by-step explanation:

$$\begin{gathered} m=400\text{kg} \\ a=20\text{cm}\to \text{m}=20:100\text{m}=0.2\text{m} \\ b=15\text{cm}\to \text{m}=15:100\text{m}=0.15\text{m} \\ h=20\text{cm}\to \text{m}=20:100\text{m}=0.2\text{m} \\ l=80\text{cm}\to \text{m}=80:100\text{m}=0.8\text{m} \\ r=2100{\text{kg/m}}_3 \\ V=(a+b)/2\cdot h\cdot l=(0.2+0.15)/2\cdot 0.2\cdot 0.8=\frac{7}{250}=0.028{\text{m}}^3 \\ {m}_1=r\cdot V=2100\cdot 0.028=\frac{294}{5}=58\frac{4}{5}=58.8\text{kg} \\ n=\lfloor m/m_1\rfloor =\lfloor 400/58.8\rfloor =6 \end{gathered}$$

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