Added problem "6. Calculate unexchangeable value" (exercism#19)

* Added new problem "Calculate unexchangeable value"

Added modular arithmetic problem Calculate unexchangeable value

* Added new problem "Calculate unexchangeable value" (example.py, exchange.py)

Modified example.py, exchange.py

* Modified Calculated unexchangeable value

Modified `unexchangeable_value()` function's expected return type to be `int`

* added new lines at the end of file

* applied prettier on instructions.md

Co-authored-by: Limm-jk <201602057@cs-cnu.org>
Co-authored-by: Junkyu Lim <57378834+Limm-jk@users.noreply.github.com>

languages/python/exercises/concept/currency-exchange/.docs/instructions.md

@@ -77,3 +77,23 @@ This function should return the maximum value you can get considering the `budge
 >>> exchangeable_value(127.25, 1.20, 10, 5)
 95
 ```
+
+## 6. Calculate unexchangeable value
+
+Create the `unexchangeable_value()` function, with parameter `budget`, `exchange_rate`, `spread`, & `denomination`.
+
+1. `budget` : amount of your money you are planning to exchange.
+2. `exchange_rate` : unit value of the foreign currency.
+3. `spread` : percentage taken as exchange fee.
+4. `denomination` : the value of a single bill.
+
+This function should return the unexchangeable value considering the `budget`, `exchange_rate`, `spread`, & `denomination`.
+
+**Note:** Returned value should be `int` type.
+
+```python
+>>> unexchangeable_value(127.25, 1.20, 10, 20)
+16
+>>> unexchangeable_value(127.25, 1.20, 10, 5)
+1
+```

languages/python/exercises/concept/currency-exchange/.meta/example.py

@@ -19,3 +19,11 @@ def exchangeable_value(budget, exchange_rate, spread, denomination):
 	actual_rate = exchange_rate + exchange_fee
 	exchangeable_amount = int((budget / actual_rate) / denomination)
 	return exchangeable_amount * denomination
+
+
+def unexchangeable_value(budget, exchange_rate, spread, denomination):
+	exchange_fee = (exchange_rate / 100) * spread
+	actual_rate = exchange_rate + exchange_fee
+	unexchangeable_amount = int((budget / actual_rate) % denomination)
+	return unexchangeable_amount
+

languages/python/exercises/concept/currency-exchange/exchange.py

@@ -1,14 +1,23 @@
 def estimate_value(budget, exchange_rate):
     pass
 
+
 def get_change(budget, exchanging_value):
     pass
 
+
 def get_value(denomination_value, number_of_bills):
     pass
 
+
 def get_number_of_bills(budget, denomination):
     pass
 
+
 def exchangeable_value(budget, exchange_rate, spread, minimum_denomination):
     pass
+
+
+def unexchangeable_value(budget, exchange_rate, spread, denomination):
+    pass
+