Lack of congruence in mathematical abilities versus writing abilities in faculty
Writing is part of your everyday life and an essential job skill, as a STEM academic. The comparison between your writing skills and the basic math skills of a humanities professor is only valid to the extent that these math skills matter for the humanities professor's job. By all means, if their mathematical inability is hindering their research or teaching, they should brush up. But frankly, I trust them to make that call, and to seek out the many resources that are already available for that purpose, if necessary. If they need "encouragement" to do this, I question whether it would be all that helpful. You could teach them to add fractions, but if they didn't use it regularly, they'd forget again. If your only reason for why they should know math is that you as a STEM academic know how to write, I think it's a stretch. It would be like a physical education professor coming to the math department and saying "I remember geometry, why can't all of you lift weights?"