How to Group By the Entitycollection records based on the column in ms crm 2011

Hi,

I have EntityCollection as mentioned below format. I need to group the records based on the column "Year" and sumup into single record.

Year        Estimatedvalue   Estimaredrevenue

2014       10                       20 

2014       30                       40

2014       50                       60

2015       70                       80

2016       90                       100

I need to get the output as below in EntityCollection

2014       90                        120

2015       70                        80

2016       90                        100 

How can i achieve this in C# plugin code. Please provide some sample code to achieve this. 

And how to append the EntityCollection records to the existing EntityCollection.

Please help its very urgent.

Thanks,

Yadav

Yadav