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Twifoo  Thank you, let me give you some context, I work for an energy company, as you may know we sometimes estimate meter readings to create a bill. we bill every 3 months - we call this a quarterly bill. If we estimate a bill and a customer calls with a meter read that is lower than we have estimated we have to amend the bill, sometimes we have to amend multiple bills if we have not taken or been given a meter reading.

For example here are the estimated readings

Q4 2019/18 - 6000

Q3 2019/18 - 5000

Q2 2019/18 - 4000

Q1 2019/18 - 3000

Q4 2018/17 - 2500

Q3 2018/17 - 2000

Q2 2018/17 - 1500

Q1 2018/17 - 1000

Q4 2017/16 - 0500

Q3 2017/16 - 0250

Q2 2017/16 - 0100

Q1 2017/16 - 0050

The customer calls and tells us the meter reading is 2200, this means we have to change the readings for all the bills that have been estimated higher than this number. in this example Q3 2018/17 is the first bill in the list that is lower than 2200 so this remains the same. We have to change all the bills above this one. We know that the Q4 2019/18 will be 2200. This means we have to change these bills

Q3 2019/18 - 6000

Q3 2019/18 - 5000

Q2 2019/18 - 4000

Q1 2019/18 - 3000

Q4 2018/17 - 2500

To calculate the estimated reads for these bills we calculate the difference between the reading for the bill that didn’t need to be changed, Q3 2018/17 and the reading the customer gave 2200. in this example the difference is 200 (2200 - 2000) and the number of bills that need to be amended is 5, giving us 40.  ( 2200-2000 / 5 )

What we are doing is working out the average usage during the period we are amending the bills and adding this average usage to each amended bill. This table explains 

Q4 2019/18 - 6000 to 2200 Changed 

Q3 2019/18 - 5000 to 2160 Changed 

Q2 2019/18 - 4000 to  2120 Changed 

Q1 2019/18 - 3000 to  2080 Changed 

Q4 2018/17 - 2500 to 2040 Changed 

Q3 2018/17 - 2000 to 2000 No Change