Level 3
A rectangular container 100 cm by 50 cm by 45 cm was 20% filled with water. A tap was turned on to fill it up with water at a rate of 9 ℓ /min. Every 30 seconds after the tap was turned on, an iron ball of volume 500 cm3 was added to the container. How many iron balls of the same volume would there be in the container when the container is 100% filled with water?
