Simple math problem

[eleanor_rigby]

I can breeze through calc but have trouble with this simple type of question. Since they would each contribute a third, I multiplied each by 1/3 then averaged them and get an answer of ~ 1.3, which is obviously wrong....can someone please take me through the steps?

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?

The right answer was 1.09 days.

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[Chokita]

I can breeze through calc but have trouble with this simple type of question. Since they would each contribute a third, I multiplied each by 1/3 then averaged them and get an answer of ~ 1.3, which is obviously wrong....can someone please take me through the steps?

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?

The right answer was 1.09 days.

This is really simple. Consider what part of the job could be done in an hour by each person. Thus, Sam could do 1/4, Lisa could do 1/6 and Tom coul do 1/2 of the job in the same amount of time. Next, find out the part of job they can together in 1 hour.
1/4 + 1/6 + 1/2 = 11/12

So, to finish the job divide 1 (100% of the job) by 11/12 = 1,09

 

[eleanor_rigby]

thanks

 

[Wow Strong Armz]

thanks

i dont have time to try it right now but kaplan said to use this formula

(a*b)/(a+b) so u would do sam + lisa then get that number and then do it with the other guy