A16: PHB (Page 190) Using Different Speeds:
"If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can’t use the new speed during the current move."

In your example, character flew 20 feet. Raw says the character can move [55-20=] 35 more feet by walking.