The solution contains 40% salt. If you add 120 kg of salt, then the solution will contain 70% salt.

The solution contains 40% salt. If you add 120 kg of salt, then the solution will contain 70% salt. What is the mass of salt in the original solution?

Mass fraction of salt in the initial solution:

40% = m (salt) / m (solution) * 100%.

Hence m (salt) / m (solution) = 0.4.

m (solution) = 2.5 * m (salt).

Mass fraction of salt after adding 120 kg of salt:

70% = (m (salt) + 120) / (m (solution) + 120) * 100%.

We substitute into this formula the expression for m (solution) obtained above:

0.7 = (m (salts) + 120) / (2.5 * m (salts) + 120).

1.75 * m (salts) + 84 = m (salts) + 120,

0.75 * m (salt) = 36. Therefore, m (salt) = 48 kg is the mass of salt in the original solution.