Start with what we know: Our lowest number is
and we want a sum, let's call that
.
We know we want consecutive numbers and we want 4 of them. Numbers are consecutive if they come "one after another", that is to say: we can get from one to the other by adding or subtracting 1.
If we start with
we can get consecutive numbers by repeatedly adding 1 (notice we aren't using the "or subtracting" part here because we know
is our lowest number).
So: starting with
, adding one over and over until we've got our four numbers we have:
And now we can write our sum:
