Stud8 probability question

[anatta]

How do you figure this out?...

You start out with 3 to a low. You are playing 8 handed. You see 4 low cards up on third street, not your rank, that is, you can catch good with 16 cards on 4th. Its pretty easy for me to figure out that its 16/42 or 38.1% that you will catch good.

My question is what does the math formula and figures look like in trying to answer the question, "how often will you make your low by 7th". You have to hit twice and you have 4 chances.