$${\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{10}} = {\mathtt{21}} = \tiny\text{Error: }$$if justin has 9 apples and 10 more apples come how many are there$${\mathtt{9}}{\mathtt{\,\small\textbf+\,}}{\mathtt{10}} = {\mathtt{21}}$$
If Justin has 9 apples and 10 more apples came, he would have 19 apples.