It takes 18 minutes to complete this section.

Then, if there is no iron block, the time required to fill the lower part of the container (the flow rate is constant during irrigation, so the volume of water is only related to time) is: 20/30* 18= 12 minutes.

Now it only takes 3 minutes to fill it up, which means that iron blocks occupy the volume of water for 9 minutes.

The ratio of the volume of the iron block to the total volume of the lower container is 9/ 12=3/4.

Whether it is a cylinder or a cuboid, the volume formula is V = S * H. Now the height H is constant, and the volume ratio is equal to the bottom area ratio, so the answer is 3/4.