Those of us playing with an Android device and thus deprived of time-traveling to earn gold, have to use our gold sparingly.  This gets harder when upgrades can only be procured with GC.  

In the 1st series of the Master level,  "Open Sky Challenge",  I was hoping to get enough trophies to open the "Euro Supercar Duel" using primarily the Caterham Seven with as few upgrades as possible.  Twenty upgrades on this car take it to a PR of 64.7, which is not enough to race in 19.3 (PR 68.0), 20.1 (PR 69.0) or 20.3 (PR 70.0).  At the same time, the futher upgrades of the Caterham could only be purchased with substantial GC.   

Without running in the 3 events requiring the KTM, or the 3 events requiring the Ariel Atom, or the 3 events mentioned above, I was still able to get enough trophies to unlock the next series.  The 1st time to date that I've moved on without getting the 75% award.