evolsteevol

This is a conditional probability issue. You should read "P(order 1 executes|order 2 executes) = 1" as the probability that order 1 executes given that you 'know for sure' that order 2 executes.

Now by common sense, and also the CFAI writes it there, "it is certain" that to satisfy the limit order condition of order 2, the price has to pass through the limit order condition of order 1. For these limit buy orders to execute, the price must keep falling from somewhere above $10 and through $9.75.

This essentially means that the conditional probability of order 1 executing, given you know that order 2 executes = 1.

"It is certain" implies probability is 1.

Hope it helps.