Question 1190016
f(4-2015)=-871+398 ln 15=206.80 million phones
254=-871+398 lnx. Isolate the ln x
1125=398 ln x
ln x=2.8266. Without rounding, raise that to the e power
x=16.889 or 17 years or 2017
check
-871+398 ln 17=256.61M which is just greater than 254M.