3:4 of 84, means "3 fourth" of 84.
To calculate " 3 fourth" of 84:
i) we divide 84 by 4, that is 84/4=21,
ii) we multiply the result of the division by 3: 21*3=63
Answer: 63 members of the choir are girls.
Remark: whenever we want to find a/b or "something":
i) divide "something" by b
ii) multiply the result by a