Order-of-magnitude analysis HW help

[kphys]

Homework Statement

1. Using an order-of-magnitude analysis, estimate the number of
breathes you will take during your lifetime.

2. Use an order-of-magnitude analysis to estimate the number of short
tons of human waste produced worldwide each year. (I did not make up
this question)

3. Using an order-of-magnitude analysis, estimate the number of
gallons of gasoline used by all automobiles in the US each year.

Homework Equations

: I don't know where to start.

The Attempt at a Solution

: #1. Counted number of breath per minute. Found out how many minutes are in a year. Calculated total breaths per year by multiplying number of breath by total minutes in a year. Then used the order of magnitue by rounding the total number down. For instance, if total number of breaths in one minute was 17, i used 10, if total number of minutes in a year is 525600 minutes, i multiplied it by 100000. Then by multiplying 17 by 525600 and 10 times 100000, I subtracted the difference. and the number is greater then 10. So i said the answer was that you can't use the order of magnitude to analyze this problem.

#2. I don't know where to start on this problem.

#3. For number 3, I'm thinking that I'll have to find the total number of cars are being used. Then multiply them by an average gallon of gas each car has. Then use that order of magnitude to by rounding down like number 1.

I probably this this wrong, so if anyone can help that would be wonderful.

 

[anorlunda]

So i said the answer was that you can't use the order of magnitude to analyze this problem.

Order of magnitude analysis does not mean that you can round each separate step to the power of 10. 6-4=2 which rounds to 1 in power of ten. But 6 rounds to 10, and 4 rounds to 1, 10-1 is 9 which rounds to 10. That's the wrong answer and the wrong analysis.