I just can't see it at the moment.
Find the sum of all positive integers less than 10,000 not divisible by seven.
Thanks guys.
From the sum of all positive integers less than 10000, substract the sum of all positive integers which are divisible by 7.
$\displaystyle \displaystyle\sum_{k=1}^{10000}k-\sum_{k=1}^{1428}7k$
and use $\displaystyle \displaystyle\sum_{k=1}^nk=\frac{n(n+1)}{2}$