user88115: First, I'm sorry if I'm asking a stupid question. I'm just trying to understand bitcoin. I'm reading the Ethereum white paper, and I couldn't understand this sentence about proof of work:

At the current target of ~2^187, the network must make an average of
  ~2^69 tries before a valid block is found

My question: how the 2^69 number of tries is computed?

Updated:
My question: why 2^69 instead of 2^70 or 2^169?

user7272: As MeshCollider's comment mentions, it is simply the difference between the total number of bits against the target bits.
As to why it is this way is just a function of probability. If you have a range of 1-100, and your target is <= 20, there are 20 numbers that satisfy your requirement. Thus, 20 * 1/100, or 20 tries out of 100 should result in a valid  number against your target.
It works the same way, although on a much larger scale here. Your target and attempt are both 256 bits numbers, which means they range from 0 to 2^256-1. If your target is 2^179, it means that 256-179, or 2^67 tries will, on average, produce a valid number against the target.

88115: <p>First, I'm sorry if I'm asking a stupid question. I'm just trying to understand bitcoin. I'm reading the <a href="https://github.com/ethereum/wiki/wiki/White-Paper#mining" rel="nofollow noreferrer">Ethereum white paper</a>, and I couldn't understand this sentence about proof of work:</p>

<blockquote>
  <p>At the current target of ~2^187, the network must make an average of
  ~2^69 tries before a valid block is found</p>
</blockquote>

<p>My question: how the 2^69 number of tries is computed?</p>

<hr>

<h2>Updated:</h2>

<p>My question: why 2^69 instead of 2^70 or 2^169?</p>
 7272: <p>As MeshCollider's comment mentions, it is simply the difference between the total number of bits against the target bits.</p>

<p>As to why it is this way is just a function of probability. If you have a range of 1-100, and your target is <code>&lt;= 20</code>, there are 20 numbers that satisfy your requirement. Thus, <code>20 * 1/100</code>, or 20 tries out of 100 should result in a valid  number against your target.</p>

<p>It works the same way, although on a much larger scale here. Your target and attempt are both 256 bits numbers, which means they range from 0 to <code>2^256-1</code>. If your target is <code>2^179</code>, it means that <code>256-179</code>, or <code>2^67</code> tries will, on average, produce a valid number against the target.</p>
