The relative density of a substance is the ratio of its density to the density of a reference substance, usually water. In this question, we are given the relative density of a mixture when two substances with densities P1 and P2 are mixed together.

1. Option 1: According to this option, the values of P1 and P2 are 6 and 2 respectively. When these substances are mixed in equal volume, the relative density is 4. When they are mixed in equal masses, the relative density is 3. This option satisfies both conditions given in the question, so it is the correct answer.

2. Option 2: This option suggests that the values of P1 and P2 are 3 and 5 respectively. However, this option does not satisfy the given conditions, as the relative density would not be 4 when mixed in equal volume and 3 when mixed in equal masses.

3. Option 3: This option suggests that the values of P1 and P2 are 12 and 4 respectively. Similar to option 2, this option does not satisfy the given conditions.

4. Option 4: According to this option, the values of P1 and P2 are 9 and 3 respectively. However, this