Let S be the number of students who studied and NS be the number of students who did not study
We can represent the total number of students (T) with the following equation
T = S + N
We are given the average scores for each group and the overall class average:
Average score for those who studied (AS): AS = 78
Average score for those who did not study (ANS): ANS = 54
Overall class average (Avg): Avg = 70
We can express the total score for all students using the average scores and the number of students in each group:
Total score for those who studied (TS): TS = AS * S
Total score for those who did not study (TNS): TNS = ANS * NS
Total score for the entire class (Total): Total = Avg * T (since Avg represents the average score per student)
Since the total score for the entire class is the sum of the scores from each group:
Total = TS + TNS
We can substitute the expressions for each score:
Avg * T = AS * S + ANS * NS
Now we can substitute the values we are given:
70 * (S + NS) = 78 * S + 54 * NS
We can rearrange the equation to isolate NS (the number of students who did not study):
20 * (S + NS) = 24 * S
20S + 20NS = 24S
-4S = 20NS
NS = (-4/20) * S (we can divide both sides by -4 as long as S is not 0, which we can safely assume since there must be at least one student who studied)
NS = -(1/5) * S
Since NS represents a number of students, it cannot be negative. Therefore, we can flip the signs to get a positive value for the fraction representing the portion of the class that did not study:
NS/T = (1/5) * S/T
We are interested in the portion of the class that did not study, so we can express this as:
Fraction of class who did not study = NS/T = 1/5.