X+Y= 102
(2/9)X=( 1/4)Y
X=54，Y=48。
A: There are 54 students in the school art group and 48 students in the computer group.
There are x girls and y boys in this school.
X=(2/5)(X+Y)+40
Y=(4/3)X
X=600，Y=800。
X+Y= 1400
There are 1400 students in this school.
3. Set up original rice X bag and original powder Y bag in the canteen.
X=Y
X- 18=(5/8)(Y-6)
X=Y=38。
Answer: There are 38 bags of rice and 38 bags of flour in the canteen.
There are x people in class a and y people in class b.
X+Y=84
(5/8)X+(3/4)Y=57
X=48，Y=36。
A: There are 48 students in Class A and 36 students in Class B..
5. Set the total length to S km.
Total round-trip time =S/3+S/6=S/2.
Total round-trip distance =2S
Average round-trip speed =(2S)/(S/2)=4.
Attendant: The average round-trip speed is 4 kilometers per hour.