Calculate the volume of a cylindrical tank with a diameter of 850 mm and a height of 1200 mm.

$$V = \pi (425 \text{ mm})^2 (1200 \text{ mm})$$ $$V = \pi (180625 \text{ mm}^2) (1200 \text{ mm})$$ $$V = 216750000 \pi \text{ mm}^3$$ $$V \approx 216750000 \times 3.1415926535... \text{ mm}^3$$ $$V \approx 680999999.99... \text{ mm}^3$$

Rounding this to the nearest whole number gives:

$$V \approx 681000000 \text{ mm}^3$$

The problem states the volume is 713 443 840 mm$^3$. There seems to be a slight discrepancy between the given volume and the volume calculated using the given diameter and height. However, following the instruction to "show how the volume was calculated if this tank has a diameter of 850 mm", we perform the calculation as above. If we assume the given volume of 713 443 840 mm$^3$ is correct, then the diameter of 850 mm might be an approximation or there's a slight error in the problem statement. For the purpose of "showing how it was calculated", we use the provided diameter and height.

Let's re-evaluate the calculation with the given volume and height to find the radius, then compare it to the 850 mm diameter.
If $V = 713443840 \text{ mm}^3$ and $h = 1200 \text{ mm}$:

$$r^2 = \frac{V}{\pi h} = \frac{713443840}{\pi \times 1200}$$ $$r^2 = \frac{713443840}{3769.911184}$$ $$r^2 \approx 189240.00$$ $$r \approx \sqrt{189240} \approx 435 \text{ mm}$$

So, the diameter would be $2 \times 435 \text{ mm} = 870 \text{ mm}$.
This indicates that the given diameter of 850 mm is inconsistent with the given volume of 713 443 840 mm$^3$ and height of 1200 mm.

Given the instruction "Show how the volume was calculated if this tank has a diameter of 850 mm", we must use the 850 mm diameter. The calculated volume is approximately $681000000 \text{ mm}^3$. The question implies that the given volume was calculated using the given diameter and height, but our calculation shows a difference. We will proceed with the calculation based on the provided diameter and height.

Using $D = 850 \text{ mm}$ and $h = 1200 \text{ mm}$:

$$V = \pi \left(\frac{850}{2}\right)^2 \times 1200$$ $$V = \pi (425)^2 \times 1200$$ $$V = \pi \times 180625 \times 1200$$ $$V = 216750000 \pi \text{ mm}^3$$ $$V \approx 681000000 \text{ mm}^3$$

The calculated volume is approximately $681000000 \text{ mm}^3$. This is how the volume would be calculated using the given diameter and height. The value $713443840 \text{ mm}^3$ provided in the question is likely based on a slightly different diameter or height.

4.3.2)

We will use the volume given in the problem statement, $V = 713 443 840 \text{ mm}^3$, as it is the stated volume of the tank.
Given: $1 \text{ litre} = 1 000 000 \text{ mm}^3$.

$$V_{\text{litres}} = \frac{713 443 840 \text{ mm}^3}{1 000 000 \text{ mm}^3/\text{litre}}$$ $$V_{\text{litres}} = 713.44384 \text{ litres}$$

$$V_{\text{litres}} \approx 713 \text{ litres}$$

The volume of the tank is approximately $\boxed{\text{713 litres}}$.

4.3.3)

Given diameter $D = 850 \text{ mm}$.

$$D_{\text{cm}} = 850 \text{ mm} \times \frac{1 \text{ cm}}{10 \text{ mm}} = 85 \text{ cm}$$

Given: $1 \text{ inch} = 2.54 \text{ cm}$.

$$D_{\text{inches}} = \frac{85 \text{ cm}}{2.54 \text{ cm/inch}}$$ $$D_{\text{inches}} \approx 33.4645669... \text{ inches}$$

$$D_{\text{inches}} \approx 33.46 \text{ inches}$$

The diameter of the tank in inches is approximately $\boxed{\text{33.46 inches}}$.