-- Look at the two numbers.
-- Say to yourself: "Self ! 3.9 is close to 4, and 5.3 is close to 5.
So the product is going to be close to (4 x 5) = 20."
-- Then do the real multiplication.
-- On your paper or your calculator, you see the digits 2 0 6 7 .
-- From your estimate (20), you know that it can't possibly be
2.067 or 206.7 or 2,067. It must be 20.67 .
