Hello s3aUsing the equationand 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:

i.e. m/sec.

And the speed of the second

i.e. m/sec

Grandad