Let H=number of hula hoops and J be the number of jump ropes.
H/J=15/4 so 4H=15J.
Now we add 8 jump ropes and h hula hoops so:
(H+h)/(J+8)=15/4 so that ratio stays the same.
Therefore, 4H+4h=15J+120.
But 4H=15J:
15J+4h=15J+120, 4h=120, h=30.
So we need 30 more hula hoops.