How to prove that the subset sum problem is polynomially reducible to the knapsack problem
The subset-sum problem is a pair $(S,k)$ and a question whether there exists $S' \subset S$ such that $\sum_{s \in S'} s = k$.
The knapsack problem is a tuple $(w,v,W, V)$ that is weights, values, the knapsack capacity and the target value, and a question can you pick such a set of items $\mathcal{I}$ such that:

$\sum_{i \in \mathcal{I}} w_i \leq W$
$\sum_{i \in \mathcal{I}} v_i \geq V$

Hint: Given an instance $(S,k)$ of subset-sum, is there a way to set the weights and values of our items in the knapsack, so that these inequalities turn into a single equality related somehow to $k$?