Question 271958
A can do 1/9 in one day and 5/9 in 5 days.
a+b=1
5/9+b=1
b=4/9
in 5 days b does 4/9 of the job 
5/x=4/9

4x=45
x=45/4=11.25
b can complete it alone in 11.25 days.
5/9+5/11.25=1

or 
d/9+d/x=1
d=5
5/9+5/x=1
5/b=4/9
check
x/a+x/b=1
a=9 and b=11.25
x=5
ok