Find the sum of all natural numbers between $200$ and $400$ which are divisible by $7$.

If the sum of $n$ terms of the series $\left(4-\frac{1}{n}\right)+\left(4-\frac{2}{n}\right)+\left(4-\frac{3}{n}\right)+\dots$ is $\frac{1}{2}\left(kn-1\right)$, then find the value of $k$.

In an $AP$, the first term is $2$, the last term is $29$ and the sum of all the terms is $155$. Find the common difference.