Nice work, that was it.
If you have a minute, I would love it if you could parse your answer. I
am a pretty basic excel user right now, and it would be neat to get a
better understanding of it's language/formulas.
Thanks again.

JMB wrote:
With your example, this formula gave me 590 for 4.2 weeks

Week1: 100
Week2: 125
Week3: 150
Week4: 175
Week5: 200

Total = (100+125+150+175+(0.2*200) = 590

Where A1 is the number of weeks and B1=25 (each weeks increase)

=SUMPRODUCT(100+((ROW(INDIRECT("1:"&ROUNDDOWN(A1,0 )))-1)*B1))+((ROUNDUP(A1,0)-1)*B1+100)*MOD(A1,1)

Does this give the results that you are looking for?

" wrote:

Can anyone help me solve a sum of series problem in excel?

For instance:
`sum 1 to n: base + (n-1 * delta)

Or more specifically:
Say I have x customers at week 1, say 100, and each week I increase my
customers by y, say 25.
Week 1 I have 100
Week 2 I have 125

And each week I sell them a cup of coffee.

How many cups of coffee total will I have sold them in say 4.2 weeks.

Week 1 100 customer = 100 cups this week, 100 total
Week 2 125 customer = 125 cups this week, 225 total
Week 3 150 customer = 150 cups this week, 375 total


What I will end up needing is how many cups of coffee from weeks 5 to
7, which I could simply subtract, but could also be made a part of the
series, which I can't figure out in excel! ;-)

Any help?
Thanks!