The number of boys 8. Hence, the number of girls is 20 – 8 = 12.

The number of boys, who have a speaking part in the play is 5. Hence, the number of boys, who do not have a speaking part in the play is 8 – 5 = 3.

The number of girls, who do not have a speaking part in the play is 7. Hence, the number of girls, who have a speaking part in the play is 12 – 7 = 5.

Hence, consider the below contingency table:

Gender Speaking part in play No speaking part in play Row Total

Boy 5 3 8

Girl 5 7 12

Column Total 10 10 20

The total number of children, who have a speaking part is 10.

Thus, if a child from the cast of the play is chosen at random, the probability that the child has a speaking part is 10/20 = 1/2 = 0.5.