Age 11 to 14 Challenge Level:
What does the word "decimal" make you think of? You could try looking it up in a dictionary.
How many hours are there in one of our days? What time is it half way through our day?
So, what time is it half way through the French decimal day? What does this tell you about the total number of hours in the French decimal day?
Do both systems have the same number of minutes in an hour?