The simple answer is that it is 1/7. The true answer is somewhat different because births are not uniformly spread across the days of the week. In some countries medical staff prefer to spend their weekends off and so there are fewer births at weekends. Then, there are other minor adjustments for the number of leap years between birth and the thirteenth birthday.
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The answer depends on where the arrow is being thrown!
The probability that exactly one will land heads up is 0.15625
If it a fair die, the probability is 1/3.
It is 0.5
0.5