Need separate sums of many continuous series

Hello,

I am having trouble figuring out what formula or series of formulas I need to do a task, or if it is even possible.

What I am working with is this:

From | To | Grade | (SUMMED GRADE)
450 | 500 | 0.5 |
500 | 525 | 0.3 |
525 | 560 | 0.7 |
590 | 610 | 0.1 |

What I would really like to do is have some formula that I can throw along an adjacent column, that will see if the previous "To" is the same as the current "From", and if it is, then to combine the grades, and for this to continue until there is a break in the series (ie., where the "From" is not the same as the previous "To"). Of course I could manually sum the values, but there are tens of thousands of entries and I really hope to avoid this.

I have tried to do an IF function, where IF the From = Previous To, then the new grade = sum of grades, and if not then it just equals the grade. However, excel does not appear to sum up equations, they have to be values, so I can't get a collective sum.

I'm feeling completely lost on this problem, are there any smart people here that can help me out? (I have attached a small sample of the data I am working with, if it makes any difference).

Thank you

I'm afraid this is not all that clear, at least to me...

Thanks for attaching the sample workbook, but...

Make sure there is just enough data to make it clear what is needed. Include BEFORE/AFTER sheets if needed to show the process you're trying to complete or automate. Remember to desensitize the data.

Click on GO ADVANCED and use the paperclip icon to open the upload window.

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