Here's how to calculate the mass percent concentration of the solution:

1. Determine the total mass of the solution:

* Mass of solute (NaCl) = 20 g

* Mass of solvent (water) = 380 g

* Total mass of solution = 20 g + 380 g = 400 g

2. Calculate the mass percent concentration:

* Mass percent concentration = (mass of solute / mass of solution) * 100%

* Mass percent concentration = (20 g / 400 g) * 100%

* Mass percent concentration = 5%

Therefore, the mass percent concentration of the solution is 5%.