Step-by-step explanation
The probability that the oldest child is a girl is given by the quotient between two numbers:
- The number of combinations where a girl is the oldest child i.e. the number of elements in the sample space that start with a G.
- The total number of elements in the sample space.
The first number is 4 since we have 4 elements starting with G: GBB, GBG, GGB, GGG. The second number is 8. Therefore the probability that we are looking for is given by:

Answer
Then the answer is 1/2.