The tank is 30% filled, meaning 100% - 30% = 70% of the tank is empty.

Now, let's express 70% as both a decimal and a fraction:

1. As a decimal:
   $70\% = \frac{70}{100} = 0.70$

2. As a fraction:
   $70\% = \frac{70}{100} = \frac{7}{10}$

So, 70% of the tank is empty, which is 0.70 in decimal and 7/10 in fraction.

Would you like more details on how this was calculated, or do you have any questions?

Here are 5 related questions:

1. If the tank is 50% filled, what percent is empty?
2. How much water is in the tank when it is fully filled?
3. How many litres of water are in the tank if it's 70% full?
4. What is the volume of water in the tank when it's 40% full?
5. If the tank is half-filled, how many litres of water are in the tank?

Tip: Remember, when converting percentages to decimals, simply divide by 100!