Task 3. A master and an apprentice need to make a concrete slab with dimensions $4 m, 3 m$ and $20 \mathrm{~cm}$, as well as 4 fence posts with dimensions $20 \mathrm{~cm}, 3 \mathrm{~m}$ and $25 \mathrm{~cm}$. The master told the apprentice to calculate and order the exact amount of concrete needed. The apprentice calculated that they need to order $30 m^{3}$ of concrete. Did the apprentice make a mistake in the calculation?
Solution. The concrete needed to make a slab with dimensions $4 \mathrm{~m}, 3 \mathrm{~m}$ and $20 \mathrm{~cm}$ is $4 \cdot 3 \cdot 0.2=2.4 \mathrm{~m}^{3}$, and for 4 pillars with dimensions $20 \mathrm{~cm}, 3 \mathrm{~m}$ and $25 \mathrm{~cm}$ is $4 \cdot 0.2 \cdot 3 \cdot 0.25=0.6 \mathrm{~m}^{3}$. Therefore, a total of $3 \mathrm{~m}^{3}$ of concrete is needed, which means the mason made an error in the calculation.