A fruit seller had some apples. He sells 40% apples and still has 420 apples. Originally, he had: 

Let the fruit seller have n apples.

According to the question,

$\begin{array}{l}\mathrm{n}\times \frac{40}{100}+420=\mathrm{n}\\ \frac{2\mathrm{n}}{5}+420=\mathrm{n}\\ 2\mathrm{n}+2100=5\mathrm{n}\\ 3\mathrm{n}=2100\\ \mathrm{n}=700\end{array}$

So, he had 700 apples Originally.