CH12: mining isn't a difficult problem

It's a simple problem whose solution requires many iterations.

ch10.asciidoc

@@ -94,9 +94,9 @@ transactions.
 and fees")))((("Proof-of-Work algorithm")))((("mining and consensus",
 "Proof-of-Work algorithm")))Miners receive two types of rewards in
 return for the security provided by mining: new coins created with each
-the block. To earn this reward, miners compete to solve a difficult
-mathematical problem based on a cryptographic hash algorithm. The
 new block (called the _subsidy_), and transaction fees from all the transactions included in
+the block. To earn this reward, miners compete to satisfy a challenge
+based on a cryptographic hash algorithm. The
 solution to the problem, called the Proof-of-Work, is included in the
 new block and acts as proof that the miner expended significant
 computing effort. The competition to solve the Proof-of-Work algorithm