Explain how using basic facts can you find 10×20×30×40 mentally?

So all you have to do is take the first numbers of each multiple and multiply them. 
1 x 2 x 3 x 4 = 24 
Now you just add the 4 zeros to your answer. 
240,000 

Our answer even checks! 
10 x 20 x 30 x 40 = 240,000 

Good Luck! :)

If so how is