Gravel is a solid, opaque, heterogenous mixture.
the solute is the one that dissolves meaning its particles are separating into the solvent, and the solvent is the one that dissolves the other substance.
Total vapor pressure can be calculated using partial vapor pressures and mole fraction as follows:

Here,
is mole fraction of A,
is mole fraction of B,
is partial pressure of A and
is partial pressure of B.
The mole fraction of A and B are related to each other as follows:

In this problem, A is hexane and B is octane, mole fraction of hexane is given 0.580 thus, mole fraction of octane can be calculated as follows:

Partial pressure of hexane and octane is given 183 mmHg and 59.2 mmHg respectively.
Now, vapor pressure can be calculated as follows:

Putting the values,

Therefore, total vapor pressure over the solution of hexane and octane is 131 mmHg.