find the relation between the focal length and the object distance

there is a specific case where for a single converging lens the object distance equals the image distance. find the relation between the focal length and the obect distance in this case. 
 

the key equation is the lens equation:
 
1/p + 1/q =  1/f    where p is the object distance, q is the image distance and f is focal length.
 
since the image distance is equal to the object distance we can write
 
2/p  =   1/f
 
so  f =  p/2