Proccess scheduling arrival time
What are their execution time and  possible start time and finish times? For each process time, its execution time is $\text{TAT}-\text{WT}$. Its start time should be WT plus one of 0, 12 and 24. Its finish time should be TAT plus one of 0, 12, 24.

$$
\begin{array}{|c|c|c|c|c|}
\hline
\text{Process} &amp; \text{WT} &amp; \text{TAT}&amp;\text{Execution Time} &amp; \text{Start Time} &amp; \text{Finish Time} \\\hline
P1 &amp; 6 &amp; 15 &amp;9 &amp; 6, 18, 30 &amp; 15, 27, 39  \\\hline
P2 &amp; 3 &amp; 6 &amp;3 &amp; 3, 15, 27 &amp; 6, 18, 30 \\\hline
P3 &amp; 63 &amp; 84 &amp; 21 &amp; 63, 75, 87&amp;84, 96, 108  \\\hline
P4 &amp; 0 &amp; 6 &amp; 6 &amp; 0&amp; 6\\\hline
P5 &amp; 24 &amp; 36 &amp; 12 &amp; 24, 36, 48&amp; 36, 48, 60\\\hline
P6 &amp; 7 &amp; 15 &amp; 8 &amp; 7, 19, 31 &amp; 15, 27, 39\\\hline
P7 &amp; 24 &amp; 39 &amp; 13 &amp; 24, 36, 48 &amp; 39, 51, 63\\\hline
P8 &amp; 6 &amp; 12 &amp; 6 &amp; 6, 18, 30 &amp; 12, 24, 36\\\hline
P9 &amp; 3 &amp; 7 &amp; 4 &amp; 3, 15, 27 &amp;7, 19, 31\\\hline
\end{array}
$$



What can we say about start times and finish times? 


The start time of any process except the first process must be the finish time of another process. 
The finish time of any process except the last process must be the start time of another process.


What does it mean by SJF? It means among all jobs that have been queued/submitted, the shortest job will be executed first.

So our task is to select 10 numbers $0, 6, \cdots$ from the column "Start time" and "Finish Time" so that


each number must appear in column "Start Time" and column "Finish Time", except the first number 0, which only appears in column "Start Time" and the last number, which only appears in column "Finish time". 
The sequence starts with 0, 6 since, as you have observed, the first process started at 0 and ended with at 6. 
For the initial numbers that are not greater than 12, their successive differences should become bigger and bigger. For the next segment of numbers that are not greater than 24, their successive differences should become bigger and bigger. For the last four numbers, their successive differences should become bigger and bigger.


There should be enough hint to set you moving again. 

Just in case you are still stuck, move your mouse over the following to reveal the spoiler.


   Check the column "Finish Time". Which process must finish last? Then which process' finish time could be the last process' start time? Then which process' finish time could be that process' start time? And so on.

 It turns out the condition SJF is not needed at all. This problem can be solved as long as the processes are executed one after another.

 On the other hand, SJF can be helpful. The three processes with longest execution time, P3, P5 and P7 must be the last three jobs since all of them started later than 24. Since SJF, $P5\prec P7\prec P3$.