In Terence Tao's book, Additive Combinatorics, page 70, it says:
For instance, if one knows that $$ A + B \subseteq A + X $$ then one can immediately deduce that $$ A + n B \subseteq A + n X $$ for all $n ≥ 0$.
I'm really confused about why this could work. Any ideas?
Adding $iB+(n-i-1)X$ to the first equation:
$$A+(i+1)B + (n-i-1)X\subset A+iB+(n-i)X$$
Therefore $A+nB \subset A+(n-1)B+X \subset \cdots \subset A+nX$