One-third of the students who originally participated in interest groups did not.

Then the ratio of people who participate in interest groups to those who don't is 1:3.

1÷ (1+3) =1/4 of the total number of sixth-grade students who participate in interest groups; Those who don't take part account for 3/4 of the total number of sixth-grade students (1+3) = 3/4.

Suppose there are x people in the sixth grade.

So 1/4x people participated in the interest group activities, and 3/4x people did not participate.

( 1/4x+30):x= 1:2

2*( 1/4x+30)=x

1/2x+60=x

x- 1/2x=60

1/2x=60

x= 120

So there are 120 students in grade six and grade one.