The new concentration will be 1.00%.
When doing percent concentration problems, you should always start by taking a closer look at your initial solution.
Your initial solution is 10.% m/v, which means that it has 10 g of sodium chloride in every 100 mL of solution. Since you have less than 100 mL of solution, you'll have less than 10 g of sodium chloride in your sample
This is how much sodium chloride your initial solution contains.
Now, you want to increase the volume of the solution to 500 mL. The key here is to realize that the amount of sodium chloride present will not change.
Your final solution will still have 5.0 g of
The volume is 10 times bigger, so the concentration must be ten times smaller.