Pipes open together: when to close some so the tank fills in a set time

The same question, asked 2 times: 10 Sep 2024 S1*, 12 Sep 2024 S2. Every wording below, oldest first.

Try each one first, then open its answer.

Question 1
10 Sep 2024 S1
60 $Three pipes P, Q and R can fill a cistern in 40 minutes, 80 minutes and 120 minutes, respectively. Initially, all the pipes are opened. After how much time (in minutes) should the pipes Q and R be turned off so that the cistern will be completely filled in just half an hour?$
Tap an option to check your answer.
Question 2
12 Sep 2024 S2
64 $Two pipes A and B can fill a tank in 48 minutes and 66 minutes, respectively. If both the pipes are opened simultaneously, then after how many minutes should pipe B be closed so that the tank gets filled in 32 minutes?$
Tap an option to check your answer.

Question 1