Tolong jawab pake cara ya, terima kasih :)

Volume air dan bola besi = volume air + volume besi.

π.(rt)2.t2 = π.(rt)2.t1 + (4/3). π.(rb)3   
(rt)2.t2 = (rt)2.t1 + (4/3).(rb)3 
36 = 756 + 108
36 = 864
t2 = 864/36 = 24

t2 - t1 = 24 - 21 = 3.