Question 250503
Since the definition of "average" is ave = sum of all scores divided by the number of scores, we have 80 =(70+86+78+fourth exaximation score)/4

So 320 = 234 + fourth examination score (multiply both sides of the equation by 4 and add the known scores)

So 320-234 = fourth examination score (subtract 234 from each side of the equation

so fourth examination must be greater than or equal to 86 in order to have an average of at 80.