The rectangular tank has the following dimensions: 50x50x42cm, its volume can be determined as follows:
![\begin{gathered} V=w\cdot l\cdot h \\ V=50\cdot50\cdot42 \\ V=105000\operatorname{cm}^3 \end{gathered}]()
To determine the volume of the stone, you have to calculate the amount of water that was on the tank before and after the stone was placed there.
Before the stone was placed, the tank was 2/3 full, to determine the amount this fraction represents you have to multiply the volume of the tank by 2/3
![105000\cdot(2)/(3)=70000\operatorname{cm}^3]()
Before the stone was placed, the tank had 70000cm³ of water.
After the stone was placed, the tank was 3/4 full, to determine how much it represents, you have to multiply the volume of the tank by 3/4:
![105000\cdot(3)/(4)=78750\operatorname{cm}^3]()
The volume of the stone is equal to the difference of the volume on the tank after the stone was placed and the volume of the tank before the stone was placed:
![\begin{gathered} V_{\text{stone}}=V_{\text{after}}-V_{\text{before}} \\ V_{\text{stone}}=78750-70000 \\ V_{\text{stone}}=8750\operatorname{cm}^3 \end{gathered}]()