Complex sentence: All unaccelerated observers agree on the amount of invariant mass in closed systems at all times, and although different observers may not agree with each other on how much relativistic mass is present in any such system, all agree that the amount does not change over time.
Split into simpler sentences: All unaccelerated observers agree on the amount of invariant mass in closed systems at all times. While different observers may not agree with each other on how much relativistic mass is present in any such system, all agree that the amount does not change over time.