Need a technical design.

Hi guys, I'm in need of a technical design. Here are the requirements. There are 20 participants who will walk through a crowded street. There's one guide who will be guiding them. The guide will be away at a distance of about 30-40 meters from the participants. Now, we require a technology by which the guide speaks in through a microphone, and all the 20 participants can hear on their headphones. What would be the cheapest and best way to do it? Please provide your technology design, the best design will be selected. Please make sure the design you propose is cheap and relialb.e