
Answer:
Boyle´s Law
Explanation:
If we increase the volume of a gas, its pressure will decrease that is why in this question the pressure was decreased to  yield the decrease in the number of collisions.
This is Boyle´s law which states that the pressure of a gas is inversely proportional to the volume of its container.
It is expressed mathematically  as PâVâ = PâVâ
where the subscripts refer to intial and final conditions.