She figured it out. What she needed to take into consideration were:

Effective oxygen binding sites, or Hill Coefficient, of haemoglobin is 2.8.

From the Hill Plot Equation for 75 Torr, we get 95.1% of O2 binds

For 20 Torr we get 32.4%

Subtract one from t'other = 62.7%

Easy, innit.

TL;DR - the additional information we needed was really just the 2.8. And that we needed to use the Hill Plot Equation.

But anyway - thanks to all who took the time to read it.