Volume (in cubic meters) = Area (in square meters) x Depth (in meters)
First, you need to convert the area from hectares to square meters. One hectare is equal to 10,000 square meters.
30 hectares = 300,000 m²
Now, convert the depth from millimeters to meters. There are 1,000 millimeters in a meter.
Depth\: = \:\frac{20mm} {1000} = \:0.02 \:meters
Now, you can calculate the volume:
Volume = 300,000 square meters x 0.02 meters = 6,000 cubic meters
Since 1 cubic meter is equal to 1,000 liters, you can convert this to MegaLiters (ML):
Volume =\frac {6,000 cubic meters} {1,000} \:= \:6 ML
So, 6 MegaLiters (ML) of water are required to irrigate a 30-hectare crop area with a depth of 20mm.