Answer:
30mm
Explanation:
The question "How much did the plant grow per day?" is asking for the rate of the corn growth for one day, in the format height/day.
The rate given is over 6 days rather than 1 day:
180 mm in 6 days
The find the rate for one day, divide the height by the days.
180mm/6days = 30mm/1day
Therefore the plant grew 30mm per day.