Ensuring task execution order in threadpool

Question

I have been reading about the thread-pool pattern and I can\'t seem to find the usual solution for the following problem.

I sometimes want tasks to be executed serial

Answer 1

How would you ensure those tasks are ordered?

push task1
push task2
push task346
push task5

In response to the edit:

push task1
push task27   **
push task3468   *
push task5
push task9