Sphere Packing

 Published on Friday, 19th December 2008, 01:00 pm; Solved by 2016;
Difficulty rating: 60%

Problem 222

What is the length of the shortest pipe, of internal radius 50mm, that can fully contain 21 balls of radii 30mm, 31mm, ..., 50mm?

Give your answer in micrometres (10-6 m) rounded to the nearest integer.