How to add hours matching a mysql query?

Data (Table name: T1):

Teacher	Subject	Day	Hour
Albert	Blue	Wednesday	10:00
Albert	Blue	Wednesday	12:00
Brandon	Red	Tuesday	09:00
Brandon	Red	Tuesday	11:00
Albert	Cyan	Monday	08:30
Albert	Cyan	Monday	10:30
Claudia	Gray	Thursday	08:00
Claudia	Gray	Thursday	10:00
Albert	Pink	Friday	13:00
Albert	Pink	Friday	14:30
Martha	Green	Wednesday	12:00
Martha	Green	Wednesday	14:00
Albert	Yellow	Friday	11:00
Albert	Yellow	Friday	12:30

As it can be seen, there is a record for the starting hour of a Subject and another for the finishing time of the same Subject (data comes like that). What I intend to know is the weekly amount of hours dedicated to classes by a specific teacher, let's say for "Albert".

Query:

$result = mysqli_query($link, "SELECT SUBTIME(max(Hour), min(Hour)) AS TeachTime, Subject FROM T1 WHERE Teachers LIKE '%Albert%' GROUP BY Subject ORDER BY Subject ASC") or die(mysqli_error($link));
if (mysqli_num_rows($result)!==0) {
echo "Weekly teaching time: ";
while($row = mysqli_fetch_array($result)){
echo $row['TeachTime']." Hrs. ";
}
}

Output:

Weekly teaching time: 02:00 Hrs. 02:00 Hrs. 01:30 Hrs. 01:30 Hrs.

Desired output:

Weekly teaching time: 07:00 Hrs.

As you can see, I don't know how to perform the addition of every resulting amount of hours. How can I achieve that?

I have also tried GROUP BY Teachers but results are weird... not the addition result.

Solution

This is the code I end up using thanks to the guidance of Bill Karwin and some more research:

$result = mysqli_query($link, "SELECT *,
SEC_TO_TIME(SUM(TIME_TO_SEC(WeekTime))) AS ProfTime FROM
(SELECT *,
SUBTIME(max(Hour), min(Hour)) AS WeekTime FROM T1
WHERE Teacher LIKE '%Albert%' GROUP BY Teacher, Subject) AS C")
or die(mysqli_error($link));
if (mysqli_num_rows($result)!==0) {
echo "Weekly teaching time: ";
while($row = mysqli_fetch_array($result)){
echo date('H:i', strtotime($row['ProfTime']))." Hrs. <br>";
}}

Bill's answer had made me realize that you can query results from an embedded query, e.g.:

Query2 over results of (Query1 over data)

Then, as results of query1 were in time format (HH:MM), I should get a total of the addition of all the instances retrieved, i.e., 01:00 + 02:00 + 01:30 = 04:30

The addition has to be performed in query2 and I have found on tutorialspoint that the query SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(Time))) AS TotalTime does that.

Finally, as the result comes in the the format HH:MM:SS and I only need HH:MM I applied a php convertion to the output: date('H:i', strtotime($row['TotalTime'])).