Assuming your plus sign means divide:
[tex] \frac{4}{10} / \frac{5}{10} [/tex] becomes
[tex]\frac{4}{10} * \frac{8}{5} [/tex] you flip the second fraction and multiply. To multiply you multiply across the bottom and then across the top.
[tex] \frac{4}{10} * \frac{8}{5} = \frac{32}{50} or \frac{16}{25} or .64[/tex]
Hope that helps. Feel free to ask any questions.
