Rule of three and Estimation Problem

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Hi!

I have a table with  Date  Store  StoreType and Cost

Table1 

Store  Date  StoreType Sales  Cost 

1                  mine         known  known

2                notmine      known  unknown

3

4

and so on  

Then I use this table to create another table by grouping sale

the table look like this 

Table 2

Sales                                         Cost

10000 (avg 5000-15000)         5000

20000 (avg 15000-25000)       7000

30000 (avg 25000-35000)       10000

40000 (avg 35000-45000)       15000

50000 (avg 45000-55000)       20000

The problem is Some Store is not mine  I only know their sales not cost

So I want to  create measure that take Sales for the mentioned Store as input and calculate thier cost base on rule of three or any suggestion.  

for example  Sales = 15000 output cost should be 6000

                     Sales = 48000 output cost should be 19000

However Table Sales is updated every month, so I cant fix number in Table 2  and  need the measure to calculate cost for other store based on my store cost  and sales  for month by month. (use JAN data for calculate JAN cost).

Thank you in advance and sorry for any mistyping.