EC: "Check" your results by order-of-magnitude estimation, back substitution, etc.

How do I know I am right?!

check the numbers

check the units

OUTCOME:

"Check" your results by order-of-magnitude estimation, back substitution, etc.

TEST YOURSELF!

If I told you there were 310 million seconds in a year, how would you know if I was telling the truth? (Without using a calculator.)