Question 778898
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Average is the total of the scores in all the tests / number of tests.
Therefore, total of scores in all tests = Average * number of tests

To get an average of 80 in 5 tests, she must score a total of 5 * 80 = 400
In the first 4 tests, she has scored a total of (83+77+81+78) = 319
So, to get a total of 400 she must score at least (400 - 319) = 81 in the final 
test. 
Lowest score she should get in the 5th test = 81.
Hope you got it :)
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