Calculate the 3rd proportional to 15 and 30.

Let the 3rd proportional = k
Then, 15 : 30 : : 30 : k
⇒ 15/30 = 30/k
⇒ 15k = 30 x 30
∴ 15k = 30 x 30
∴ k = (30 x 30) / 15 = 60