Final answer:
Approximately 20% of interest groups in Washington, D.C. are public-interest groups, serving broader public concerns rather than specific economic interests.
Step-by-step explanation:
About 20 percent of the interest groups represented in Washington, D.C., are public-interest groups. Public interest groups are organizations that represent the concerns of the public, such as health, environment, and consumer protection issues, instead of economic or corporate interests. In comparison to private interest groups that seek particularized benefits, public-interest groups aim to promote benefits that serve the broader public good.