If Lidia buys 15 apps for an average of $4 each, then the total cost of the apps is 15 * $4 = $60.
If she has $66 for this purpose, then she will have $66 - $60 = $6 left after purchasing all the apps she needs.
#### 6
The answer is: 6
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
If Lidia buys 15 apps for her new tablet, each costing an average of $4, and she has $66 for this purpose, how much money will she have left after purchasing all the apps she needs?

### Answer:
