First, you need to find out how much the cyclist will travel a single day. Since the cyclist travels 16 km/h and for 8 h/day, you can easily multiply 16 x 8 = which would be 128 km/day.
To find how long it will take the cyclist to travel 800km, just divide 800 by the amount they will be going in one day. 800/128= 6.25. That would convert to 6 days and 6 hours.