Calculate the mass of salt in 100 g of a solution with a mass fraction of 0.2.

To begin with, we write down an expression to determine the mass fraction of a substance in a solution:
w (in-va) = (m (in-va) / m (solution)) * 100%.
We have w (in-va) given not in percent, so there is no need to multiply by 100%.
w (in-va) = (m (in-va) / m (solution))
Let us express the mass of a substance from this expression:
m (in-va) = (w (in-va) * m (solution)).
Substitute the numerical values:
m (in-va) = (w (in-va) * m (solution)) = 0.2 * 100 = 20 g.
Answer: the mass of salt in solution is 20 g.