This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Let's race to the forty-second floor!
Koichi owns a forty-two-story condominium in Tokyo.
It’s pretty annoying that there’s a lot of bureaucratic red tape involved in moving, and to make matters worse, sometimes the person in charge has an attitude problem. However, the worst part of my moving this time was that the elevator was out of service when I moved in and my apartment was on the forty-second floor.