Update a dynamic number of rows with an amount proportionally Since my English are not perfect I will try explaining with an example

I have tableA that I want to update the Amount 

groupID reservationID  Date        Amount
26      1              2018-06-01  NULL
26      2              2018-06-01  NULL
26      3              2018-06-01  NULL
27      4              2018-06-04  NULL
27      5              2018-06-04  NULL


And tableB that holds the total amount per Group

groupID  Date        Amount
26       2018-06-01      60
27       2018-06-04     100


I trying to find a way that 


groupID 26 amount of 60 can updated the first table
groupID26 rows with 20 each (60 / 3)
groupID 27 amount of 100 can update the first table groupID27 rows with
50 each (100/2)


In other words a query that will be smart enough to divide the total amount in tableB  the in the number of rows in tableA where: 

tableA.Date = tableB.Date
AND tableA.groupId = tableB.groupID


Thanx

Here is a sample code ddl

DECLARE @tableA TABLE
(group_id       numeric(10,0)   null,
 reservation_id numeric(10,0)   null,
 ondate         datetime        null,
 amount         numeric(10,2)   null)

DECLARE @tableB TABLE
(group_id       numeric(10,0)   null,
 ondate         datetime        null,
 amount         numeric(10,2)   null)

INSERT @tableA
SELECT 26,1,'2018-06-01',NULL
INSERT @tableA
SELECT 26,2,'2018-06-01',  NULL
INSERT @tableA
SELECT 26,3,'2018-06-01',  NULL
INSERT @tableA
SELECT 27,4,'2018-06-04',  NULL
INSERT @tableA
SELECT 27,5,'2018-06-04',  NULL

INSERT @tableB
SELECT 26,'2018-06-01',60
INSERT @tableB
SELECT 27,'2018-06-04',100
You can solve it this way:

declare @a table 
(groupID int, reservationID  int, [Date] date, Amount decimal(10,2));

insert into @a values 
(26,      1,              '2018-06-01',  NULL),
(26,      2,              '2018-06-01',  NULL),
(26,      3,              '2018-06-01',  NULL),
(27,      4,              '2018-06-04',  NULL),
(27,      5,              '2018-06-04',  NULL);

declare @b table 
(groupID  int, [Date] date,  Amount decimal(10,2));

insert into @b values
(26,       '2018-06-01',      60),
(27,       '2018-06-04',     100);

with cte as
(
select a.*,
       b.Amount / count(a.reservationID) over (partition by a.Date, a.groupId) new_amount
from @a a join @b b 
        on a.Date = b.Date
        AND a.groupId = b.groupID
)

update cte
set Amount = new_amount;

select *
from @a;