(2)25- 18=7 (person)
12- 10=2 (person)
12-8=4 (person)
20- 15=5 (person)
7>5>4>2
Therefore, the number of people participating in the calligraphy group in the two grades is the closest.
The total number of people participating in the art troupe is: 12+8=20 (people)
(3) 18+25=43 (person)
10+ 12=22 (person)
12+8=20 (person)
15+20=35 (person)
43>35>22>20
Because the number of people who participate in the computer group is the largest, the computer group is the most popular among the fifth and sixth grade students.
So the answers are: (2) Calligraphy, 20; (3) Computer group, because the students who participated in the computer group in grade five or six were the most.