Answer:
4 hours
Step-by-step explanation:
The amount remaining at the end of each hour is 0.85 of the amount at the start of the hour. You want to find the exponent such that ...
... 0.85^x = 0.5 . . . . . x = the number of hours until half is left
... x·log(0.85) = log(0.5) . . . . . take the log to turn it to a linear equation
... x = log(0.5)/log(0.85) ≈ 4.265024
Rounded to the nearest hour, x = 4.
