You overhear a conversation.

1. One person says

$10\in \mathbb{N}$ and $3\in \mathbb{N}$, and $10-3=7\in \mathbb{N}$. In general, if $m\in \mathbb{N}$ and $n\in \mathbb{N}$, then $m-n\in \mathbb{N}$

1. Another person says

Brilliant! Also, $0.8\notin \mathbb{Z}$ and $0.7\notin \mathbb{Z}$, and $0.8+0.7=1.5\notin \mathbb{Z}$. Generally, if $a\notin \mathbb{Z}$ and $b\notin \mathbb{Z}$, then $a+b\notin \mathbb{Z}$.

Prove that they are both wrong.

We haven't spoken about how to prove things in mathematics yet, but that doesn't matter for this example - all you need to do to prove each person wrong is find a single example where the claim fails.