Question 910638
a=5 b=8 c=6
1/a+1/b+1/c=1/x
1/5+1/8+1/6=1/x
48/240+30/240+40/240=1/x
118/240=1/x
240/118=x

or 2.03389831 hours working together
check
1/a+1/b+1/c=1/x
1/5+1/8+1/6=1/2.03389831
48/240+30/240+40/240=1/2.03389831
118/240=1/2.03389831
240/118=2.03389831
2.03389831=2.03389831
ok