exam Maths Puzzle Numerical Ability

I am wondering if I can get help with this particular question. I have write it in exponential form.
What I got for this was
4_____
1 / 2 , but it was wrong

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exam Other Question
If Sam can do a job in 4 days that Lisa can do in 6 days and Tom can do in 2 days, how long would the job take if Sam, Lisa, and Tom worked together to complete it?
thank you for all your help need help bad

1/z+1 - 6-z/6z = 0