I attached the question and answer explanation below.

I think there's an error in the answer explanation so it should be Q1' + Q2' = Q1

Conceptually, why is the sum of the new charge magnitudes for both capacitors C1 and C2 equal to the original charge magnitude Q1 for just C1?

I understand their "quicker" way of thinking about this question, but I'm not understanding their first method of solving it mathematically through Q1' + Q2' = Q1

I think there's an error in the answer explanation so it should be Q1' + Q2' = Q1

Conceptually, why is the sum of the new charge magnitudes for both capacitors C1 and C2 equal to the original charge magnitude Q1 for just C1?

I understand their "quicker" way of thinking about this question, but I'm not understanding their first method of solving it mathematically through Q1' + Q2' = Q1