Question 866752
x = time it takes for Dave to do the job alone
y = time it takes for Mike 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 = 20
y = unknown (leave it as y for now)
z = 4



Plug those into the equation and solve for y



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



{{{1/20 + 1/y = 1/4}}}



{{{1/y = 1/4 - 1/20}}}



{{{1/y = 5/20 - 1/20}}}



{{{1/y = (5 - 1)/20}}}



{{{1/y = 4/20}}}



{{{1/y = 1/5}}}



{{{1*5 = y*1}}}



{{{5 = y}}}



{{{y = 5}}}



So it will take <font color="red">5</font> hours for Mike to do the job alone.