Weekly Problem 12 - 2010
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 $80$%.