Answer: 8 days.
Step-by-step explanation:
To find how many days Billy needs to work, we can first subtract what he has ($50) from what he needs ($170) to get:
170 - 50 = 120
Next, we can divide 120 by the number of dollars he can earn each day ($15) to get:
120 ÷ 15 = 8
Therefore, he must work 8 days.
Hope this helps :)