Give the step-by-step reasoning process and then the final answer. Marta works on her grandparent's farm to raise money for a new phone. So far, she has collected $240. For every hour she works, she receives $10. Her grandmother often gives her tips, and she has collected $50 in tips in total. How many hours has Marta worked on the farm so far?
Ignoring tips, Marta earned $240 - $50 = $190. Therefore, she has worked 190 / 10 = 19 hours.
The final answer: 19.