1st post walks 5 m + 5 m = 10m
2nd post walks 10m + 10m = 20
Arithmetic sequence d = 10 a1=10 a200 = 2000
sum n(a1+a200)/2 = 200 (10+2000)/2 = 201000 In reality, she doesn't need to walk back after the last post is placed
so subtract 1000 200 000 = 200 km