The eqn reduces to [10 + 10*y/(x+y)]

This implies k >10 but <20.. Now it all depends on what the term 10*y/(x+y)...
3 possibilities

a) If y = x then k = 15
b) if x > y then k <15 and from the answer choices 12 would be the answer then
c) if x < y then k > 15 and in which case from answer choices u have 18

So , is this question complete ? . Is there is no info if x > y or x<y or anything ?

With the question as it is there is no one answer till u know x<y or x>y or x= y

Sorry, x<y, but you already got the right answer