Solution: Two-fifths of the participants in the Chinese contest and three-quarters of the participants in the math contest, so there are both. Therefore, the extra part of the addition of the two is that both of them participated in the calculation once more, so both of them participated in 2/5+3/4- 1=3/20 of the total number of people in the competition.

Because both have 12 participants, the total number of participants is 12÷(3/20)=80.

Because the number of participants accounts for 40% of the whole grade, the number of students in the whole grade is 80/40% = 200.

There are 200 students in the whole grade.