The density would be 10.5
It seems to overall decrease.
Answer:
Explanation:
A homogeneous mixture is a mixture in which the composition is uniform throughout the mixture. The salt water described above is homogeneous because the dissolved salt is evenly distributed throughout the entire salt water sample.
For this question, lets apply Avagadro's law
when Pressure and temperature are constant, the volume occupied is directly proportional to the number of moles of gases.

where V-volume, n-number of moles and k - constant
Therefore at 2 instances

where V1 and n1 are for 1st instance
and V2 and n2 are for 2nd instance
therefore

V1 = 2.4 L
n1 = 3.7 mol
n2 = 3.7 + 1.6 = 5.3 mol
since more He moles are added at the 2nd instance its the sum of the moles.
V2 needs to be calculated

V2 = 2.4 x 5.3 / 3.7
= 3.4 L
Answer is 1st option 3.4 L