+12 Carissa the chessplayer is a very, very slow walker. In fact, she walks at $100$ meters per hour when walking uphill, $120$ meters per hour when walking across flat ground, and $150$ meters per hour when walking downhill. One day, Carissa walks across Boston from a café to a boba shop, and then takes the same route in reverse to return to the café. What was Carissa's average speed during the entire round trip?
