30 minutes have been taken off You face a reduction of 30 minutes In these sentences time is either added or taken away, so an amount of time is taken off and you reduce time by an amount of. We would use 'in' to say that the exam begins in 30 minutes, or the train arrives in 30 minutes, or the program finishes in 30 minutes. You have 30 minutes less - this is correct.

"30 minutes have been taken out you have 30 minutes less you face a reduction in 30 minutes for you to complete the exam" Of for these sentences, we usually say them this way: "30 minutes have passed" or "It has been 30 minutes since the exam has started." Hope this helps!