The average score of girls is above: (92.5-9 1) times 24=36.
Every boy is below average: 9 1-89=2.
The total score of boys is lower than the average = the total score of girls is higher than the average.
So the number of boys; 36/2= 18
Method two.
Let the number of boys be x.
Then (9 1-89)X=(92.5-9 1) times 24.
X= 18
Method three.
Let the number of boys be x.
Then 89X+92.5 times 24=9 1 times (X+24).
X= 18