If you break down the beginning part, its 150 ÷ 3 = 50, so that means each class took 50 minutes. Not only that, you get your 'Time/Minutes per Class'. From there, you can multiply 50 by 5 and get 250 minutes, or 4 hours and 10 minutes.
In short: 250 minutes or 4 hours and 10 minutes.