Question 1161494
If r for the rate per hour of any 1 man, 
x number of hours each day for the question,


Number of total work hours would be  (workhours/day)*(numberofdays);


{{{10r*8*30=1=8*r*x*40}}}


To solve for x,
{{{(8*40rx)/(10*8*30r)=1}}}
.
.
{{{x=7&1/2}}}
They work seven and a half hours each day.