Find the velocity of the ball?(answer is 5 m/s. How to find it?)

Horizontal square shaped platform of side 10 m on the ground has initially sides coinciding with x-axis and y-axis as shown. A platform starts moving along x-axis with a velocity 10 m/s. A person standing on the platform starts moving on platform towards diagonally opposite corner as shown, with a velocity (5 times square root of 2) m/s. A stationary person standing at origin (Not on the platform) throws a ball in air so that the ball hits the person on platform when it is exactly at corner of the platform. Find the velocity of the ball?(answer is 5 m/s. How to find it?)