The cost of the mangoes is 6 mangoes × $3/mango = $<<6*3=18>>18.
The cost of the apple juice is 6 cartons × $3/carton = $<<6*3=18>>18.
Find the total cost of the groceries which is $18 + $18 = $<<18+18=36>>36.
Rihanna has $50 − $36 = $14 left.
#### 14