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!