That be arithmetik sequens
with start=7, step size=3, n=n
Sum av ALL such sequenses...sum=n*(start number+lastnumber)/2
or sum=n*averaej...averaej=(start+end)/2....& end=start+(n-1)*stepsize
zampel...start=7, step=3, n=12 get ansers...sum=282, end number=40,
start+end=47, averaej=23.5