Thanks a lot,both of you,now I know it.

Nobody says dozens of minutes or tens of minutes. How cruelly bad some advice can be! Thank God for Jmat, who gave you the correct answer. Give your answer in terms of 20/30/40 minutes.

They're both technically correct, but you're unlikely to hear them much outside when you're using maths. It's be much more natural to say "About half an hour/about 30/40/45 minutes"/about 30 to 45 minutes".

yes you can both of them