5.1% - 0.051
Decay formula = total = start value x (1-rate)^time
200 = 320 x (1-0.051)^t
200 = 320 x 0.949^t
Divide both sides by 320"
0.625 = 0.949^t
Take the logarithm of both sides:
log 0.949^t = log 0.625
t * log 0.949 = log 0.625
t = log 0.625/log 0.949 = 8.98 minutes
Rounded to nearest minute = 9 minutes
