Answer:
1/1
1.0
100%
Step-by-step explanation:
N = 1 corresponds to day 1
n = 2 corresponds to day 2
...etc etc...
n = 21 corresponds to day 21
Plug in n = 21
a_n = 100 + (n-1)*4
a_21 = 100+(21-1)*4 ... replace every n with 21
a_21 = 100 + 20*4
a_21 = 100+80
a_21 = 180
Answer: C) 180 yards