Question 866236
x = time it takes for Jack to do the job alone
y = time it takes for Marilyn to do the job alone
z = time it takes for them to get the job done when they work together. This is assuming that one person doesn't slow the other down and they work in the most efficient way as a team.



When we make those definitions above, it turns out that we can tie x,y,z together with this equation 


{{{1/x + 1/y = 1/z}}}



In this case, 



x = 7
y = 5
z = unknown (leave it as z for now)



Plug those into the equation and solve for z



{{{1/x + 1/y = 1/z}}}



{{{1/7 + 1/5 = 1/z}}}



{{{5/35 + 1/5 = 1/z}}}



{{{5/35 + 7/35 = 1/z}}}



{{{(5 + 7)/35 = 1/z}}}



{{{12/35 = 1/z}}}



{{{12*z = 35*1}}}



{{{12z = 35}}}



{{{z = 35/12}}}



{{{z = 35/12}}}



So it will take {{{highlight(35/12)}}} hours for them to do the job if they work together. This is assuming that one person doesn't slow the other down and they work in the most efficient way as a team.



Since {{{35/12 = 2 & 11/12}}}, this means it takes {{{highlight(2 & 11/12)}}} hours (<font color="red">2 hours, 55 minutes</font>)



If you want the time in minutes only, then multiply it by 60 to get {{{(35/12)*60 = 175}}} minutes



Note: the time it takes, in hours only, as a decimal is approximately 35/12 = <font color="red">2.91666666666667 hours</font>