Question 703212
 Mary had an average score of 78 points over x number of tests.
 How many marks must she score for her next test so that she can get an average of 80 points?
:
Let t = the score required to raise the av to 80
:
{{{((78x + t))/((x+1))}}} = 80
multiply both sides by (x+1) 
78x + t = 80(x+1)
78x + t = 80x + 80
t = 80x - 78x + 80
t = 2x + 80, is the scored required to get an 80 av