SELECT
    SUM(CASE WHEN a.Type = 1
                THEN CAST(DATEDIFF(s,
                                ca.StartTime,
                                ca.EndTime) AS FLOAT)
                    / 60.0
                ELSE NULL
        END) AS WorkingTime ,
    AnnotherResult ...

Select ... , something/WorkingTime, something*WorkingTime, ...
From
(
Select *, CASE WHEN a.Type = 1
                THEN CAST(DATEDIFF(s,
                                ca.StartTime,
                                ca.EndTime) AS FLOAT)
                    / 60.0
                ELSE NULL
        END) AS WorkingTime
from myTable
) t

DECLARE @CA TABLE
(
    StartTime datetime,
    EndTime datetime,
    ActivityType int,
    Employee varchar(50)
);
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-01 1:00 PM', '2014-04-01 2:00 PM', 1, 'Jim');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-03 1:00 PM', '2014-04-03 3:00 PM', 1, 'Jim');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-04 9:00 AM', '2014-04-04 11:00 AM', 1, 'Jim');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-05 10:00 AM', '2014-04-04 11:13 AM', 0, 'Jim');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-02 10:00 AM', '2014-04-02 3:00 PM', 1, 'Sally');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-04-04 1:55 PM', '2014-04-04 3:11 PM', 1, 'Sally');
INSERT INTO @CA (StartTime, EndTime, ActivityType, Employee) VALUES ('2014-03-30 1:45 PM', '2014-04-01 1:00 AM', 0, 'Sally');

-- Method #1: CTE

WITH BaseData AS
(
    SELECT
        Employee,
        SUM (
                CASE
                    WHEN ca.ActivityType = 1 THEN
                        1
                    ELSE
                        0
                END
            ) AS NumType1Entries,
        SUM (
                CASE
                    WHEN ca.ActivityType = 1 THEN
                        CAST(DATEDIFF(s, ca.StartTime, ca.EndTime) AS FLOAT) / 60.0
                    ELSE
                        NULL
                END
            ) AS TotalMinutesType1
    FROM        @CA ca
    GROUP BY    Employee
)
SELECT
    BaseData.Employee,
    BaseData.NumType1Entries,
    BaseData.TotalMinutesType1,
    CASE
        WHEN BaseData.NumType1Entries > 0 THEN
            BaseData.TotalMinutesType1 / BaseData.NumType1Entries
        ELSE
            NULL
    END AS MinutesPerType1Entry
FROM        BaseData;

-- Method #2: Derived Table

SELECT
    BaseData.Employee,
    BaseData.NumType1Entries,
    BaseData.TotalMinutesType1,
    CASE
        WHEN BaseData.NumType1Entries > 0 THEN
            BaseData.TotalMinutesType1 / BaseData.NumType1Entries
        ELSE
            NULL
    END AS MinutesPerType1Entry
FROM        (
                SELECT
                    Employee,
                    SUM (
                            CASE
                                WHEN ca.ActivityType = 1 THEN
                                    1
                                ELSE
                                    0
                            END
                        ) AS NumType1Entries,
                    SUM (
                            CASE
                                WHEN ca.ActivityType = 1 THEN
                                    CAST(DATEDIFF(s, ca.StartTime, ca.EndTime) AS FLOAT) / 60.0
                                ELSE
                                    NULL
                            END
                        ) AS TotalMinutesType1
                FROM @CA ca
                GROUP BY Employee
            ) AS BaseData;

-- Method #3 - Cross Apply

SELECT
    Employee,
    SUM (Calcs.Type1EntryCounter) AS NumType1Entries,
    SUM (Calcs.Type1EntryTotalMinutes) AS TotalMinutesType1,
    CASE
        WHEN SUM (Type1EntryCounter) > 0 THEN
            SUM (Type1EntryTotalMinutes) / SUM (Type1EntryCounter)
        ELSE
            NULL
    END AS MinutesPerType1Entry
FROM        @CA ca
CROSS APPLY (
                SELECT
                    CASE
                        WHEN ca.ActivityType = 1 THEN
                            1
                        ELSE
                            0
                    END AS Type1EntryCounter,
                    CASE
                        WHEN ca.ActivityType = 1 THEN
                            CAST(DATEDIFF(s, ca.StartTime, ca.EndTime) AS FLOAT) / 60.0
                        ELSE
                            NULL
                    END AS Type1EntryTotalMinutes
            ) AS Calcs
GROUP BY    Employee;

-- Method #4 - Inline Table-Valued User-Defined Function, Deterministic

-- Commented-out because it can't be run without creating the function first.

/*

SELECT
    Employee,
    SUM (Calcs.Type1EntryCounter) AS NumType1Entries,
    SUM (Calcs.Type1EntryTotalMinutes) AS TotalMinutesType1,
    CASE
        WHEN SUM (Type1EntryCounter) > 0 THEN
            SUM (Type1EntryTotalMinutes) / SUM (Type1EntryCounter)
        ELSE
            NULL
    END AS MinutesPerType1Entry
FROM        @CA ca
CROSS APPLY dbo.CalcType1Statistics
            (
                ca.ActivityType,
                ca.StartTime,
                ca.EndTime
            ) AS Calcs
GROUP BY    Employee;

*/

-- Method #5 - Inline Table-Valued User-Defined Function, Non-Deterministic Aggregation

-- Commented-out because it can't be run without creating the function first.
/*

SELECT
    EmpSummary.Employee,
    EmpSummary.NumType1Entries,
    EmpSummary.TotalMinutesType1,
    CASE
        WHEN EmpSummary.NumType1Entries THEN
            EmpSummary.TotalMinutesType1 / EmpSummary.NumType1Entries
        ELSE
            NULL
    END AS MinutesPerType1Entry
FROM    dbo.GetEmployeeSummary () AS EmpSummary

*/

SELECT WorkingTime
    FROM (
           SELECT SUM(CASE WHEN a.Type = 1 THEN CAST(DATEDIFF(s, ca.StartTime, ca.EndTime) AS FLOAT) / 60.0
                           ELSE NULL
                      END) AS WorkingTime .....)

;WITH WT AS (SELECT SUM(CASE WHEN a.Type = 1 THEN CAST(DATEDIFF(s, ca.StartTime, ca.EndTime) AS FLOAT) / 60.0
                               ELSE NULL
                          END) AS WorkingTime .....)
SELECT WorkingTime
FROM WT
JOIN .....