I need help figuring how to handle a running total with two different multipliers

I have a chart that tracks a stacking bonus for a weight loss competition. For each month a contestant loses weight, they get $10 per pound for that month. If they have a weight loss streak, they also get $5 per pound they've lost for the previous months. I've come up with a simple, but not perfect solution and could use some help. I'm using Excel 2010.

I'll attach a copy but basically I have the sheet setup so it has a running total cell (sum of all weight loss cells) then every fourth cell has a sum of that period's loss. The formula I use subtracts the period loss amount from the total loss amount, then multiplies the result by 5. Then it adds that to the result of the period loss amount multiplied by 10.

The problem is as more data gets added in the following 4 week periods, the previous 4 week period's reward amount continues to change as well.

I've put a comment to have the user just manually enter the calculated result into the reward field after the calculation has been made, but I know there has to be a more elegant solution.

Thanks in advance for the help,
Thom

P.S. No this isn't a homework problem. My company is coming up with an awesome health bonus initiative. It is amazing what the threat of rising premiums can do...

Oh wow. That looks great! Thanks for the help, Benishiryo! I would have never thought of that.

@ supakeepa

Based on your last post it seems that you are satisfied with the solution(s) you've received but you haven't marked your thread as SOLVED. I'll do that for you now but please keep in mind for your future threads that Rule #9 requires you to do that yourself. If your problem has not been solved you can use Thread Tools (located above your first post) and choose "Mark this thread as unsolved".
Thanks.

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Thanks, Cutter. I was looking for how to close this question, but couldn't find it. I did give Benishiryo rep points