Basically, you need to divide these by the number of minutes it took to make them. So, for example:
Since one hour and a half equals 90 minutes, Steve baked cookies at a rate of 0.61111...... (the one keeps repeating) per minute (55 divided by 90).
Since one hour equals 60 minutes, Vondra baked cookies at a rate of 0.516666...... ( the six keeps repeating) per minute (31 divided by 60).
This means that Steve baked batches of cookies at a greater rate than Vondra.