1. Find the total mass of the solution:

* Total mass = mass of solute (NaNO3) + mass of solvent (water)

* Total mass = 4.2 g + 60 g = 64.2 g

2. Calculate the mass percentage:

* Percent concentration by mass = (mass of solute / total mass of solution) x 100%

* Percent concentration by mass = (4.2 g / 64.2 g) x 100%

* Percent concentration by mass ≈ 6.54%

Therefore, the percent concentration by mass of the solution is approximately 6.54%.