CONCEPT: -Age problem
-Always come down to current age of subject(s) in such questions

SOLUTION: Let Bina's current age =x
Let Arti's current age=y
Then (x-5)=3(y-5)=3y-15=>x=(3y-10)....(i)
Also,(x-10)= (1/2)*(c-10) =>x=(c-10)/2+10...(ii)
Using value of x in (i),
 =>y=c/6+5 (B)