If ∑n=1∞arn−1=20\sum_{n=1}^{\infty} a r^{n-1} = 20∑n=1∞arn−1=20 and a=5a = 5a=5, what is rrr?
0.750.750.75
0.250.250.25
0.50.50.5
0.80.80.8