Chapter P, Prerequisites - Section P.9 - Modeling with Equations - P.9 Exercises - Page 70: 24

82.5%

We know that 6 students scored 100, 3 students scored 60 and the rest (25-6-3=16 students) scored a mystery amount ($x$). The average of the 25 students is 84: $\displaystyle \frac{6*100+3*60+16*x}{25}=84$ $600+180+16x=84*25=2100$ $16x=1320$ $x=\frac{1320}{16}=82.5$ So the last 16 students must have an average score of 82.5%.