Find the least value of n for which
How do I do this?
To be honest, I would but I don't know how I'd explain it. We've learned how to calculate all terms in a sequence, how to find a, d, n and l. And this is our first look at the summation notation. If there are other methods, and we've been taught it, it should hit me when I see it.
The common difference between each term is 4. Therefore d = 4.
There are n terms.
Substitute all that into your formula for the sum of an arithmetic series and then do with the result what I said to do in my first reply.