Probability of Finding a Block
To look at it a different way, the probability for solving a block in Bitcoin is approximately:

1 / ({total network hash per second} x {average block time in seconds})


Note that block time is the average.

As of writing, this is:

Probability = 1 / (24,667,167,000,000,000 x 600)
= 1 / 1.48003002×10¹⁹
= 6.75661970694351186200939356621969059789746697164966964... × 10^-20 (WolframAlpha)


Data source: blockchain.info