from my double maths
EPE (elastic potential energy / work done from natural length) = lambda x^2 /2l
i've attached a spreadsheet
first, work out value of the spring force thing (grr can't remember proper name)
even better you can work out lambda/2l
from the working you can see you get this to be 2/144
then use this value to work out the work done in extending the spring from the point with no extention, to x=5 and x=10, and work out the difference
i get 25/24, or
1.041667 
Hope this helps