I like to reduce everything by half if I don't already know the answer.

It just so happens that is the reduced fraction.
All I did was halve 18, which is 9, and halve 34, which is 17.
Answer:
Step-by-step explanation:
x - y = 34
x + y = 212
-------------------add
2x = 246
x = 246/2
x = 123 <=== 123 domestic
x + y = 212
123 + y = 212
y = 212 - 123
y = 89 <===== 89 foreign
Let y=total monthly income
Let x=each time app is opened
y=25+(.003x)