Hello, kangta27!

. .

Choose any of the 30 available numbers.

Cross out that number and any number in its row and in its column.

This leaves a array of numbers.

Choose any of the 20 available numbers.

Cross out that number and any number in its row and in its column.

This leaves a array of numbers.

Choose any of the 12 available numbers.

It seems that there are: . ways.

But theof the selection of the three numbers is not considered.order

Hence, we must divide by

Answer: . ways.