1. Jun 25, 2014

### Bijackar

1. The problem statement, all variables and given/known data

many generations have passed since the year 0 AD?

Let me first say that this is my first time taking physics and this is all new to me so I appreciate all the help.

2. Relevant equations

here is the answer I found by googleing but I don't quite understand it. I was wondering if someone could explain it to me.

history x (10^11s / history) × (1 generation / 1/3 lifetime) × (0.5 lifetime / 10^9s) = 150

3. The attempt at a solution

How I approached this problem was that I chose 70 to be an average person's lifetime then I divided that by 3 to find what a generation is and then divide 2015 by that number which is not what the answer says. I think I just don't get the question.

2. Jun 25, 2014

### SammyS

Staff Emeritus

What time interval are they assuming for the "history" of whatever?

What time interval are they assuming for a "lifetime" ?

3. Jun 25, 2014

### Bijackar

4. Jun 25, 2014

### SammyS

Staff Emeritus
Look at their expression.

(1011s / history) is a conversion factor for units of "history" to seconds.

How many years are there in 1011 seconds?

Similarly, (0.5 lifetime / 109s) is a conversion from seconds to a "half a life(time)".

How many years are in 109 seconds?

5. Jun 26, 2014

### TheAustrian

A "Generation" is commonly 25 years. The Question is asking how much (unknown variable) multiplied by 25 gives 2015?

6. Jun 26, 2014

### SammyS

Staff Emeritus
By the way, I should have said:

Welcome to PF!

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