You will be given a definition of a task first, then some input of the task.
The task is to generate the correct answer for a given math problem.

Problem: the spherical ball of lead 3 cm in diameter is melted and recast into 3 spherical balls . the diameters of two of these are 1 1 / 2 cm and 2 cm respectively . the diameter of third ball is ?
Output:
2.5