A committee of the five is to be chosen from a group of people. The probability that a certain married couple will either serve together or not at all is

$\frac{4}{9}$

From a group of 9 people, 5 people can be chosen in ${^9C}_5$ ways.

∴ Total number of ways of forming the committee = ${^9C}_5$

The number of ways in which a certain married couple is either in the committee or it is not included in the committee, is 

${^7C}_3 × {^2C}_2 + {^7C}_5$

Hence, required probability = $\frac{^7C_3+{^7C}_5}{^9C_5}=\frac{4}{9}$