Prove that for all $n\ge9$, there exist natural numbers $x,y$ such that $n=2x+5y$. How would you use induction to prove this? 

If $n=9$ then $n = 2\cdot 2 + 5\cdot 1$; taking $x:= 2$ and $y:=1$ suffices.
If $n \geq 9$ is an integer such that
$n-1= 2x + 5y$ for some integers $x,y > 0$, then
$n = n-1 + 1 = 2x+5y + 1 = 2x' + 5y'$.
Note that $2x+5y+1 = 2x+5y+(5-4) = 2(x-2) + 5(y+1)$.
So the preceding equalities are equivalent to
$$
n = 2x' + 5y' = 2(x-2) + 5(y+1).
$$
Taking $x' := x-2$ and $y' := y+1$ suffices,
which are still integers.
Note that for $n=10$ we have $x' = 2-2 = 0$, which is not a natural number if you don't count $0$ as one such.
