There are few Questions about Percentage and Average that need to solve and would like to know general formula's to solve such questions.Questions are:

Q:1: If the price of petrol be raised by 12.5%, find by what percentage should a consumer decrease the consumption of petrol so that there is no change in his monthly expenditure on petrol?

Q:2: In an examination 80% of the students passed in Maths and 70% passed in English while 10% of the students failed in both the subjects.If 360 students passed in both the subjects, find the total number of students who appeared in the examination?

Q3: As rise of 20% in the price of refined groundnuts oil a family is forced to buy i kg less for $180.Find the original price of refined groundnut oil per kg.

Q:4: Nine students of a class contribute a certain sum. Seven of them give $5 each and the other two give $5 and $9 more than the average contribution of the class of 9 students is?

Q:5: A batsman has a certain average of runs for 11 innings. In the 12th inning he scores 129 runs, thereby increasing his average by 9 runs. What is his average for the first 11 innings?