Дано

w(NaCl) = 20% = 0.2

m(NaCl) = 6.4 г

Решение

w(NaCl) = m(NaCl) / (m(NaCl) + m(H2O)) = 6.4 / (6.4 + m(H2O)) = 0.2

6.4 / (6.4 + m(H2O)) = 0.2

6.4 = 0.2(6.4 + m(H2O))

6.4 = 0.2(6.4 + m(H2O))

6.4 = 1.28 + 0.2m(H2O)

6.4 = 1.28 + 0.2m(H2O)

5.12 = 0.2m(H2O)

m(H2O) = 25.6 г

Ответ: 25.6 г