There is an unknown number of people at the beginning of lunch. Then... Each student had 3 friends come sit with them, tripling the amount.
Then the entire group divided into 4 groups, and all but one group left and went outside, so only 1 fourthof the total students were left in the lunchroom
7 more people came into the lunchroom
The total number of people in the lunchroom at the end of lunch was 16