Answer
P(one child under the age of 18) $=\frac{353}{1700}\approx0.208$
Work Step by Step
The sample space: 1700 married couples. So, N(S) = 1700
According to the marginal distribution (see definition, page 235) of the second column: N(one child under the age of 18) = 353.
P(one child under the age of 18) =
$\frac{N(\text{one child under the age of 18})}{N(S)}=\frac{353}{1700}\approx0.208.$
