And because Xiao Ning arrived at school after receiving the book in 10 minutes,

So for Xiao Ning, it's 50 minutes' journey from home to school.

So in the 40th minute, Dad caught up with Xiao Ning.

It took Dad 10 minutes to catch up with Xiao Ning.

So dad set out to chase Xiao Ning at 40- 10=30 minutes.