5 * 4 / (100 + 15 * 3) ≈ 0.138
As an extra example, in 1st place with the same conditions, you normally have a 20% chance of getting a Shield from Special Item Boxes. With Defense Item Chance UP, this is raised to exactly 50%!
20 * 4 / (100 + 20 * 3) = 0.5
5 * 4 / (100 + 15 * 3) ≈ 0.138
As an extra example, in 1st place with the same conditions, you normally have a 20% chance of getting a Shield from Special Item Boxes. With Defense Item Chance UP, this is raised to exactly 50%!
20 * 4 / (100 + 20 * 3) = 0.5