How can i extact this output using linq

Question

SCHEID   ACTIVITYID   PID  Quantity Amount
2   12   100 200     2000
2   12   101 300   2500
2   15   102 400   1400
2   15   103 120   2000
This is what i managed to get from join query using three tables in linq, how can i add quantity, amount depending on ActivityID, and i want to display both activityid value which belongs to same scheme
Please Help me out....
ie, i whant output .....
 Scheme  ActivityID    Quantity Amount
2   12    500   4500
2   15   520  3400