Day 15 Something to Think about

Hi! The question says,"You bike up a hill at 4 miles per hour and come back down the same way at 12 miles per hour. What is your overall average speed for the entire roundtrip?"
I'm confused of why the average speed isn't equal to all the speeds mentioned / the number of speeds. Like, if you're driving on a highway and for the 1st second your speed is 100 km/h, for the 2nd second your speed is 102km/h and for the 3rd second your speed is 106 km/h, your average speed would be 102km/h which you get by doing (100+102+106)/3 =102 km/h. Following this logic, wouldn't the average speed equal (12+4)/2= 8 miles per hour?