Answer
$P(at$ $least$ $20)=\frac{6327}{6427}\approx0.984$
If we randomly select 1000 mothers involved in a multiple birth we would expect about 984 to be a mother who was at least 20 years old.
Work Step by Step
N(S) = 6427 and N(15-19) = 100. So:
$P(15-19)=\frac{N(15-19)}{N(S)}=\frac{100}{6427}$
The event "mother who was at least 20 years old involved in a multiple birth" is the complement of "mother 15 to 19 years old involved in a multiple birth". So:
$P(at$ $least$ $20)=$ $1-P(15-19)=1-\frac{100}{6427}=\frac{6427}{6427}-\frac{100}{6427}=\frac{6327}{6427}\approx0.984$
