Explanation:
4a.
as we can see in the table, if we have 1 processor and add a second one, we save 30 minutes (61 - 31 = 30).
4b.
if you solved 3. and filled the table correctly, you see that with 240 processors we need 1 minute + 60/240 = 1 1/4 = 1.25 minutes.
if we add additional 240 processors, we get 480 processors, and we need 1 minute + 60/480 = 1 1/8 = 1.125 minutes.
that means we save 1/8 minute (1 1/4 - 1 1/8 = 1/8) with doubling the amount of processors. that is 7.5 seconds.
4c.
the answer always depends on the circumstances.
in fast processes 7.5 seconds can be a long time, and it might be worth the effort to save that amount of time (like in real time processing as with industrial robotics, simulating and controlling trains or a space flight).
but for asynchronous calculations of certain numbers it might not matter, if the result is available 7.5 seconds earlier or later, and the extra effort is not worth it.