Question

Imagine an attacker has amassed 20% of the network's hash power, and is looking to trick someone by double spending a 1-confirmation transaction.

He sends 10btc to Bob, Bob waits for 1 confirmation, and sends the attacker some other cryptocurrency in exchange.

Now the attacker creates an alternative transaction that spends the same coins to himself instead, and has to create a block containing his double-spend transaction, as well as another block based on that previous one, in order to create a 'longest chain' containing his double spend.

What is his probability of succeeding? Enter answer in percents, accurate to two decimal places.

Answer 1