Let M=number of men and W=number of women. M+W=150 persons.
The total weight of the men is 70M kg and that of the women is 55W kg.
The total weight of all 150 people=70M+55W.
The mean weight of one person is therefore (70M+55W)/150=60.
W=150-M, so:
70M+55(150-M)=9000,
70M+8250-55M=9000,
15M=750,
M=50 so there are 50 men and 100 women.