Formula with grouping in Crystal Report

I have a problem that I could not solve to generate a report in crystal report with c# .

Assuming I have the following table in sql server

 - A B C D
 - G 3 2 1
 - G 5 6 7 
 - Y 2 5 6 
 - Y 3 2 3 

Which I am interested in generating crystal report grouped by A and generating a new column using a formula in crystal report of which the new column E is going to be the total sum and division by group between column B and C

Example Group G:

B: 3 + 5 = 8 
C: 2 + 6 = 8
E: 8 / 8 = 1

Example Group Y:

B: 2 + 3 = 5 
C: 5 + 2 = 7
E: 5 / 7 = .71

So the report would be grouped by column A:

 - A B C D E
 - G 8 8 1 1
 - Y 5 7 6 .71

I applied the formula in crystal report for the grouping of A {Command.B / Command.C} Which is wrong since I do the division for each record of the same group and I will add it.

I am looking for you to first do all the sum of B and all the sum of C and finally divide it, how do I apply it in a formula of crystal report ?