u1 = 125
u6 = 1/25
A)
125 r^5 = 1/25
r^5 = (1/25) * (1/125)
r^5 = (1/ 5^2) * (1/5^3)
r^5 = 1/ 5^3 take the 5th root
r = 1/5
B)
125 ( 1 - (1/5)^n) / ( 1-1/5) < 156.22
125 ( 1 - (1/5)^n) / (4/5) < 156.22
156.25 ( 1 - (1/5)^n) < 156.22
1 - (1/5)^n < 156.22/156.25
1 - 156.22/156.25 < (1/5)^n { 1/5 = .2 }
3 / 15625 < .2^n take the log of both sides
log (3/15625) < n log (.2)
Divide both sides by log (.2)......this results in a negative.....so....reverse the inequality sign
log (3/15625) / log (.2) > n
5.3 > n
n = 5
C) 125 ( 1 - (1/5)^n) / (4/5) = 160
156.25 ( 1 - (1/5)^n) = 160 (1)
Note that ( 1 -(1/5)^n) CANNOT be any larger than 1
So ....as large as the left side of (1) can be is 156.25