Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Let the distance between the two children be d. Also let the time taken by sound to reach the other child via air be t1 and via aluminium rod be t2.
Velocity of sound in air va = 340 m/s
∴ t1 = d/va = d/340. (1)
Velocity of sound in aluminium rod vR = 6420 m/s
∴ t2 = d/vR = d/6420. (2)
Dividing equation (1) and (2), we get
t1/t2 = 6420/340 = 18.88