ANSWER
100 miles
Step-by-step explanation
The map legend says that 2 inches on the map equals 50 miles.
Towns A and B are 4 inches apart on the map.
Let the real distance be x.
We can compare these distances like this:
2 inches = 50 miles
4 inches = x miles
Cross-multiply:
2 * x = 4 * 50
Divide through by 2:

Towns A and B are 100 miles apart.