adds memento mori

learning_python3.md

@@ -772,6 +772,40 @@ Also think about how you can *break* this program!
 
 # Coding challenge - Memento Mori calculator
 
+[Memento Mori](https://en.wikipedia.org/wiki/Memento_mori) is a bit of a grim concept and if it freaks you out, maybe adjust the exercise to calculate the days until your next birthday.
+That being said, we all die and we all live in a country with a life expectancy.
+The [Belgian](https://www.healthybelgium.be/en/health-status/life-expectancy-and-quality-of-life/life-expectancy) life expectancy for a man is about 80 years so if you know your birthday, which you should, you can calculate your theoretical death day.
+Plus, you can calculate the percentage of your life that remains.
+
+<details>
+  <summary>Spoiler warning</summary>
+
+```python3
+import datetime
+
+life_expectancy_years = 80.8
+life_expectancy_days = life_expectancy_years * 365
+delta = datetime.timedelta(days=life_expectancy_days)
+
+date = input("what is your birthday? (example: 1986 10 7) ")
+year, month, day = date.split()
+
+birthday = datetime.date(int(year), int(month), int(day))
+memento_mori = birthday + delta
+
+days_to_live = memento_mori - datetime.datetime.now().date()
+percentage = (days_to_live.days / life_expectancy_days) * 100
+
+print("your theoretical death day is:", memento_mori)
+print("that's", days_to_live.days, "left to live")
+print("which means you have {:.2f}% left to live...".format(percentage))
+```
+
+</details>
+
+There are quite a few quirky tricks in the code above.
+Take your time to ask me about them, or use online documentation and keep some notes.
+
 # Writing your first library
 
 TODO import pretty print and digital dice