8) B

Average mass of first group = 75 kg
Total mass if group of N people = 75 x N = 75N

Three people join the group. Total number of people now = N + 3
Average mass now = 78
Total mass now = 78 (N + 3) = 78N + 234

Average mass of the three people = 90 kg
Total mass of three new people = 90 x 3 = 270 kg

Total mass of new group = Total mass of initial group + Total mass of three new people

78N + 234 = 75N + 270
78N – 75N = 270 – 234
3N = 36
N = 12