How much salt and water by weight is needed to prepare 25 g of a 0.5% solution?

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%.
Let us express the mass of a substance from this expression:
m (in-va) = (w (in-va) * m (solution)) / 100%.
Substitute the numerical values and find the mass of the salt:
m (salt) = (w (salt) * m (solution)) / 100% = 0.5 * 25/100 = 0.125 g.
m (solution) = m (in-va) + m (H2O).
Let us express the mass of water from here and substitute the numerical values:
m (H2O) = m (solution) -m (in-va).
m (H2O) = m (solution) -m (salt) = 25-0.125 = 24.875 g.
Answer: you need to take 24.875 g of water and 0.125 g of salt.