PL/SQL :: Select Exists On Time

I have this sample:
the column data1 is datetime datatype  with
t as (   select 'SMITH' nom,to_date('21/09/2013 07:30:00') data1
from dual union all   select 'ALLEN',to_date('21/09/2013 07:40:00')
from dual union all   -- select 'WARD',to_date('21/09/2013 07:50:00')
from dual union all   
select 'JONES',to_date('21/09/2013 08:00:00')
from dual union all   

[Code]..

How can I write a select to check that If I input 10 minutes to nom 'ALLEN' it's ok because the time 07:40 + 10 minutes = 07:50 the row not exists, (the next)but If input 20 it exists because the sum = 08:00 and row  isn't free , indeed, there is 'JONES'?