jmc

In 5 days, Gage skated for $5\times 75=375$ minutes, and in 3 days he skated for $3\times 90=270$ minutes. So, in 8 days he skated for $375+270=645$ minutes. To average 85 minutes per day for 9 days he must skate $9\times 85=765$ minutes, so he must skate $765-645=\boxed{120}$ minutes = 2 hours the ninth day.