### How do I know I am right?!

check the numbers

- back-substitution (recalculate using answer)
- order-of-magnitude (approximate the math and do it in your head
- are you within a factor of 10?)
- reasonableness - did you get a negative time or mass? a volume
bigger than the ocean?

check the units

- is the equation dimensionally valid?
- does you answer have the correct dimension for what you
**expected** to calculate?

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.)