Marlin was saving half-dollars. He started with 10 half-dollars in his container. He adds 2 half-dollars each day. How many half-dollars will he have on the 15th day? Choose the rule that matches this situation.
1. 10 + 2 x 2
2. 10 + 15
3. 15 + 10 x 2
4. 10 + 2 x 15