What surprises me is the reasoning path she took (ie estimating customer behavior). I'd have tried to guess how many servers they need for that (uuuuh...100,000?) and how much they cost a year ($1000 each, $100m?) and what's an acceptable profit margin (20%? So about $300,000/day in earnings).
Then again you could figure Google makes about $5 billion a year in profit, say a tenth of that comes from gmail, and the profit margin is 10%, suggesting ~$15m a day in earnings (not profits) which is off by a factor of 500 from the above. That sounds a lot more likely.
To be honest, I think the approach she took is a lot more reasonable. The whole point of building a product business is that your revenue scales differently from your expenses. Your example has an implicit assumption that Google is somehow artificially regulating demand so they can keep their profit margin at 20%. The profit margin on Gmail could be 5% or 99%. Who knows?
No argument here. The only reason I went down this road was because I figured any guesses I made about actual click rates of users would be mostly based on a sample of 1 and thus even more laughably off than guessing the minimum viable earnings...of course, this is why one should never be ashamed to say you don't know. It's one thing to speculate and exhibit your thinking process, but when you start getting confident about it it's all too easy to slip in bullshit artistry, and I'm sure they'd rather avoid that trait in hiring.
Then again you could figure Google makes about $5 billion a year in profit, say a tenth of that comes from gmail, and the profit margin is 10%, suggesting ~$15m a day in earnings (not profits) which is off by a factor of 500 from the above. That sounds a lot more likely.