Then there is: 0.4 * m1(m1+5) = 0.3.

The solution is m 1 = 15.

If 60% alcohol solution of m2 kg is added, the concentration will become 50%.

So there is: (15 * 0.4+0.6 * m2)/(15+5+m2) = 0.5.

The result is m2 = 40.

So add 40 kilograms of 60% alcohol solution, and the concentration will become 50%. Let the mass of 40% alcohol solution be m 1 kg.

Then there is: 0.4 * m1(m1+5) = 0.3.

The solution is m 1 = 15.

If 60% alcohol solution of m2 kg is added, the concentration will become 50%.

So there is: (15 * 0.4+0.6 * m2)/(15+5+m2) = 0.5.

The result is m2 = 40.

So add 40 kilograms of 60% alcohol solution, and the concentration will become 50%.