To find the mass of the salt in the solution, we first need to calculate the mass of water.

Given:
m (раствора) = 400g
m2 (H2O) = 100g

We know that the mass of the solute (salt) and the mass of the solvent (water) make up the total mass of the solution:
m (раствора) = m1 (соли) + m2 (H2O)

Substitute the given values into the equation:
400g = m1 + 100g

Rearrange the equation to solve for m1 (mass of salt):
m1 = 400g - 100g
m1 = 300g

Now we need to calculate the mass percent of the salt (ω1) in the solution:
ω1 = (m1 / m) 100
ω1 = (300g / 400g) 100
ω1 = 0.75 * 100
ω1 = 75%

Therefore, the mass percent of the salt in the solution is 75%.