General Subjects: For a pneumatic transmission system for instrumentation purposes, if a pneumatic...

The regulator range is 3-15 psi, a difference of 12 psi. So, since 3 psi is the minimum an input of 9 psi would actually be 50% (9 psi minus 3= 6 psi). And 50% of 1000 is 500 rpm. What they're talking about is a pneumatically controlled flow valve. One of these:
http://www.lesliecontrols.com/iomindex/20_1_5_2Rev3.pdf

I wish all the questions had the formula's.