Answer: they bought 36 postcards.
Explanation:
Jeany bought 12 postcards which was twice as many postcards as joe bought. It means that the number of postcards that Joe bought is
112/2 = 6 postcards
Ruth bought 3 times as many postcards as joe. It means that the number of postcards that Ruth bought is
3 × 6 = 18 postcards
Therefore, the number of postcards that they bought together would be
12 + 6 + 18 = 36 postcards