Answer : We know that,

Probability of occurrence of an event

Let B be Boy and G be Girl

Total possible outcomes are BB, BG, GB, GG

Our desired outcome is at least one boy. So, BB, BG, GB are desired outputs. Total no.of outcomes are 4, and the desired outcomes are 3

Therefore, the probability of at least one boy

Conclusion: Probability of at least one boy is 2/7