When you are working on more task on the same time, you will spend on a task more time than, if you were working on tasks sequentially.

When tasks are done sequentially, time that is spent on a task is equal task length:

Task	T1	T2	T3
Time	4	10	6

When tasks are not done sequentially, time that is spent on a task is more than task length:

Task	T1	T2	T3
Time	18	20	6

Conclusion:

More time is spent on a task, more could be the cost for completing the task.