Question 356737
Let x = #students who took the test. Then the sum of all scores in the test without her score is 80x.  Plus her score, the total sum would be 80x + 100.
Dividing by x+1 (as there would be x +1 students now), 
{{{(80x + 100)/(x+1) = 81}}}.
{{{80x + 100 = 81x+81}}}, after cross-multiplication.
{{{100-81 = 81x-80x}}},
{{{x = 19}}}, the number of students who took the test.