2014 KIMC keystage3-IWYMIC individual Question 8

In the parallelogram $ABCD$, $\angle BAD=60^\circ$. $E$ is the midpoint of $BC$ and $F$ is the midpoint of $CD$. $BD$ intersects $AE$ at $M$ and $AF$ at $N$. If $AB=15$ cm and $AD=8$ cm, what is the length, in cm, of $MN$?