Add all the multiples of 11 that are less than 400. What sum do you get?

The multiples are 11, 22, ..., 396 (since $396=11\times 36$ while 407 is already above 400), so $n=36.S=\frac{36}{2}$$(11+396)=18(407)=7326$. The value 6930 uses $n=35$, one term too few. 7733 wrongly includes 407 as a 37th term. 14652 leaves out the factor $\frac{1}{2}$.