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Math Help - Need Help in Basic maths

  1. #1
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    Need Help in Basic maths

    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?
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  2. #2
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    There aren't general formulae for problems like this. You just have to think about them. Do you know how to find a percentage of a quantity?

    Now there are general formulae for some of the specific quantities that may arise in problems. For example, the general formula for the average of a set of quantities a_1,a_2,a_3....a_n is just:

    Average=\frac{a_1,a_2,a_3....a_n}{n}
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