Probability and genetic inheritance

A number of people are stopped at random in the street. They are asked if they have two children and also if one of them is a boy born on a Friday. Someone answers yes to both questions. What is the probability that this person has two boys? (Equal chance of giving birth to a boy or girl and of giving birth on any day).

Answer: 13/27

How do I solve this question? Preferably with probability formulas.
Thank you!

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