Average maths prob help

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Hi - Can anyone Expert explain in simple step how to work out pls! 1)The average mark of a student in Mathematics is 14/20.If the sum of the marks is 84, how many marks are included in average? 2)The average of 2 exams of a students is 17/20.If the difference between the 2 marks is 6, what are the 2 marks? 3) Mona recieved an average grade of 16 on her 1st 3 exams.On the 4th exam her garde was 14.What must Mona obtain in her 5th so that her overall average is 15? Many txs in advance:-)