It's not possible to pay tuition fees, rent, food and other living costs on 20 hours per week except you're lucky enough to be making $50 an hour or something. I was blessed enough that my dad was able to pay for my tuition and rent throughout my schooling so it wasn't always an issue. Although I have occasionally worked more than the 20 hours, maybe 24 hours a week and then 16 hours the next week, sometimes things happen that you get scheduled for more hours.
However, I do know people who have worked full time (35-40+ hours), whilst in school which you're not supposed to do other than during a scheduled break (e.g. during the summer). The only risk with this is when you're applying for a PGWP they might reject you because you violated the terms of your study permit as they have a way of checking how many hours you've worked except you were being paid under the table (cash). Again, some people have been able to get away with it, some haven't.
I won't judge or police anyone and tell them you must only work 20 hours or less because I know how hard and costly it is to live here, especially without having strong financial sponsorship backing you. I would only say be careful how you go about it especially if you plan on staying here long term (i.e. getting your pgwp, PR)
I have always thought the 20 hours a week rule is unfair but you have to keep in mind that if you're on a study permit you're here to study and not work. Plus they're assuming you can afford the costs of being here when you applied.
Your respond really help a lot . You sound realistic . Thanks.
Anyway am still waiting for my visa approve i sumitted on April going for September intake. Been hoping for the best