Gantt chart question 168 in PPI Practice exam

Hi all,

This is question 168 from the PPI practice exam. For the description "Steel erection begins once the concrete work is complete and is projected to take six weeks." From my understanding, this means the steel should start after the concrete is done, but from the answer it looks like these two tasks have overlap.

Meanwhile, for the duration of the window and door installation task, I didn't find where does the description indicate it will be around 6 weeks as shown in the answer.

I attached the question and answer page here. Thanks to anyone who could help in advance!