To find out how many miles the cyclists biked each day during the last 4 days of their vacation, we first need to determine how many miles they biked during those 4 days.
- Total miles biked: 287 miles
- Miles biked in the first 3 days: 131 miles
Next, we can calculate the distance biked during the last 4 days:
\[ \text{Miles biked in the last 4 days} = \text{Total miles} - \text{Miles in the first 3 days} = 287 - 131 = 156 \text{ miles} \]
Now, since they biked the same distance each day during the last 4 days, we divide the total miles biked during those days by 4:
\[ \text{Miles per day} = \frac{\text{Miles biked in the last 4 days}}{4} = \frac{156}{4} = 39 \text{ miles/day} \]
Therefore, the cyclists biked 39 miles each day during the last 4 days of their vacation.