Answer :
you could estimate by doing 600x3=1800
(becasuse of the 0's in 600 it is easier to do 6*3=18 and then add two 0's to the end 1800)
then you simply multiple 640x3=1920
(becasuse of the 0's in 600 it is easier to do 6*3=18 and then add two 0's to the end 1800)
then you simply multiple 640x3=1920