An airline parks all of its planes at Sydney airport or Melbourne airport overnight.
The transition diagram below shows the change in the location of the planes from night to night.
There are always `m` planes parked at Melbourne airport.
There are always `s` planes parked at Sydney airport.
Of the planes parked at Melbourne airport on Tuesday night, 12 had been parked at Sydney airport on Monday night.
How many planes does the airline have?
- 25
- 37
- 62
- 65
- 85