how to solve accumulative issue

[#1]

hi,

the records are look like this:

d

branch	doctor	days from today	free time minutes
a	a	1	15
a	a	1	30
a	a	2	90
a	a	3	60
b	c	1	60
b	c	2	30
b	c	2	40
b	d	4	90

by branch and doctor I need to show the average(avg)  of the closest X free turn larger then Y minutes

for example for

X=3

Y=30

doc a in branch a its (1+2+2)/3=1.66 (1=its for the second row,2+2 its the next two chunks of 30 min from the row of 90 minutes free.

for doc c in branch b its (1+1+2)/3=1.3

for doc d in branch b its (4+4+4)/3=4

X and Y are input fields!!

and can be changed by the user online.

so whay needed is accumulative calculations on the fly on this records...

to get the results like this:

branch	doctor	average free day from today
a	a	1.66
c	b	1.3
b	d	4

you can imagine that the real data is thousands of doctors and like handers of branches...

thanks for any help

Doron