Statistical mechanics, bernoulli's forumula - collision freqency

[AirTycoon]

Hello,
I am having trouble understanding collision frequency in following discussion from Kubo,

$$collision\ frequency = \frac{1}{\frac{l}{v_{x}}}=\frac{p_{x}}{ml}$$

What am I missing ?

 

[jtbell]

Between two collisions against the same wall, a molecule must travel a round-trip distance equal to twice the width of the container.

 

[AirTycoon]

ah. I was counting the opposite wall too. Thank You.

ps. Should I close this thread now ?