# Determining execution times of a concurrent system with exclusive resources

A certain system consists of 2 CPUs. The CPU that is not being used is assigned to a task whose execution is requested. On this system, two tasks, A and B, are executed.These tasks both use a common resource R exclusively.The CPU usage of the tasks A and B, the usage of resource R and the execution sequence are shown in the figure below.if both tasks are started at the same time, how long in ms will the completion of processing take for the two tasks?

a)120

b)140

c)150

d)200

According to this question, I understand the resource R is used exclusively by the CPUs, mean if one is using it, the other has to wait.

So my answer is d :

10 + 50 (first cpu) + 50(second cpu) + 60(first) + 30 (second) = 200


But the given answer is b, 140 :

10 + 50 + 60 + 20 = 140 ms


I don't get it, could someone explain to me?

Any help is greatly appreciated!