Hello s3aand taking when the first ball is thrown, the height of the first ball above the roof at time seconds is given by:, takingThe height of the second ball above the roof at time is:
They meet when ; i.e. when:
So, as you say, they meet after seconds.
When ,So the meet m below the level of the roof.
Using , the speed of the first ball is:
And the speed of the second