It turns out that wearing a simple surgical face mask dramatically reduces the detection of coronavirus in a closed laboratory setting over a period of 30 minutes (1). It also turns out that social distancing alone works too (2). Both reduce the R0 (reproduction number) of a disease. Masks reduce the "contagiousness" because fewer infectious droplets make it out into the open. Social distancing and quarantine measures reduce the likelihood of infectious transmission between infected and healthy individuals.
This is multiplicative.
For the sake of easy numbers, let's assume that wearing a mask captures 90% of infectious droplets and that every meter of distance results in 60% of the droplets in the air to fall to the ground. If a sick individual wearing a mask coughs out 100 starting droplets and is 2 meters away from a healthy individual, the healthy individual will be breathing in 2.5 infectious droplets (100 * 0.10 * 0.50^2).
This is multiplicative.
For what it's worth, in real-world studies, when the use of face masks and/or social distancing became enforced, the rate at which infectious transmission occurred saw a measurable drop. The introduction of mandates requiring face masks (with social distancing already in effect) saw a 2% decline in the daily growth rate of COVID (3). If you'd like to read more, there is a recent meta-paper that surveys 172 different studies and concludes that:
"The findings of this systematic review of 172 studies (44 comparative studies; n=25 697 patients) on COVID-19, SARS, and MERS provide the best available evidence that current policies of at least 1 m physical distancing are associated with a large reduction in infection, and distances of 2 m might be more effective. These data also suggest that wearing face masks protects people (both health-care workers and the general public) against infection by these coronaviruses, and that eye protection could confer additional benefit. However, none of these interventions afforded complete protection from infection, and their optimum role might need risk assessment and several contextual considerations. No randomised trials were identified for these interventions in COVID-19, SARS, or MERS." (4)
1: https://www.nature.com/articles/s41591-020-0843-2
2: https://www.thelancet.com/journals/laninf/article/PIIS1473-3...
3: https://www.healthaffairs.org/doi/10.1377/hlthaff.2020.00818
4: https://www.thelancet.com/journals/lancet/article/PIIS0140-6...