Self doubt

Question

Consider the processes P1, P2, P3 arrived in the sequence P2, P3, P1 and the burst time of the
processes are 3, 3, 24 respectively. What is the average TAT?
(a) 30 (b) 20
(c) 40 (d) 35

Comments

Scheduling used??

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Not mentioned

Not getting any of the mentioned options, getting 10 using FCFS.
But Answer =20

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if scheduling algorithm not given, just left the problem, don't worry to assume more things, note that it is not standard question.

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so if nothing is mentioned lets assume it to be based on FCFS.

process	AT	BT	CT	TAT (CT-AT)
p1	0	3	30	30
p2	0	3	3	3
p3	0	24	27	27
AVG...->				(30+3+27)/3=20

GANTT CHART : 

p2(3)

p3(27)

p1(30)

so avg. tat : 20 units.