Congratulations, NASA has selected you to visit the planet Zog but before you go, you’d better learn something about their currency. Today’s puzzle is courtesy of The Ultimate Mathematical Challenge.

The currency used on the planet Zog consists of bank notes in three denominations, differing only in colour. Three green notes and eight blue notes are worth 46 zogs; eight green notes and three blue notes are worth 31 zogs. How many zogs are two green notes and three blue notes worth?