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Stage: 3 Short Challenge Level: Challenge Level:1

After her second test her average mark is $\frac{6}{10}$, after her third test her average is $\frac{11}{15}$, after her fourth it is $\frac{16}{20}=\frac{4}{5}=80$% exactly and after the fifth it is $\frac{21}{25}=84$%.

So she needs to take four more tests to obtain an average mark of more than $80$%.

This problem is taken from the UKMT Mathematical Challenges.