[c0brac0bra]

A UCLA study 'found that a mental health “concern” affected 78% of the unsheltered population and a substance abuse “concern,” 75%.' [0]

[0] https://capolicylab.org/health-conditions-among-unsheltered-...

[tristor]

78% and 75% are pretty close to 80%, it may not be an exact Pareto distribution, but it's "close enough for government work".