## Help with this formula

Cost should be calculated on a category based . and for each category different calculation method i.e.
from reading A to B on one month is 25 pt./meter
from reading B to C on one month is 35pt./meter
First example if this is the first time to caculate the cost and reading is 45 KW, Category A starts from 0 and Category B starts from 50 then caculation will be as follows:
45 KW* 25 pt = 1,125 pt
Second example if this is the first time to caculate the cost and reading is 100 KW, Category A starts from 0 and Category B starts from 50 then caculation will be as follows:
50 KW* 25 pt + (100 KW-50 KW) * 35 pt = 3,000 pt
Third example if this is the second time to caculate the cost and reading is 150 KW, Category A starts from 0 and Category B starts from 50 and last reading of previouse month was 100 KW. then caculation will be as follows:
This month reading = 150-100 = 50 KW
50 KW* 25 pt =1,250 pt

So its simple. But what if Category A is chosen 50, and Category B is chosen 100. How would the calculation become?