I finished the problem set!!

I did change the test_numberwords.py since the words for the tens
place didn't actually match the number, e.g. 43 was originally
written as "fifty-three" there. It was interesting to see the test
cases. I tried to test some of my own numbers as I was writing, but
I missed the case of numbers ending in zero. 60 becomes
"sixty-zero", which matches what test_numberwords.py was looking for,
but in real life people would be puzzled by the extra "-zero." This
higlights for me an important point about making sure cases to be
tested cover all the sort of edge cases.

numberwords.py

@@ -15,12 +15,17 @@ TENS_NAMES = [
 ]
 
 def int_under_1000000_to_str(number):
-    pass
+    check_number_in_range(number, 0, 1000000)
+    if number < 1000:
+        return int_under_1000_to_str(number)
+    return int_under_1000_to_str(number // 1000) + " thousand " + int_under_1000_to_str(number % 1000)
 
         
 def int_under_1000_to_str(number):
-    pass
-
+    check_number_in_range(number, 0, 1000)
+    if number < 100:
+        return int_under_100_to_str(number)
+    return DIGIT_NAMES[number // 100] + " hundred and " + int_under_100_to_str(number % 100)
 
 def int_under_100_to_str(number):
     check_number_in_range(number, 0, 100)
@@ -39,9 +44,9 @@ def check_number_in_range(number, minimum, maximum):
     Raises an error if the number is not in range.
     """
     if number < minimum:
-        raise ValueError("{number} must not be below {minimum}.")
+        raise ValueError(f"{number} must not be below {minimum}.")
     if number >= maximum:
-        raise ValueError("{number} must be less than {maximum}.")
+        raise ValueError(f"{number} must be less than {maximum}.")
 
 def divide_with_remainder(dividend, divisor):
     """Divides one number by another, using whole-number division. 

test_numberwords.py

@@ -12,17 +12,17 @@ test_cases = [
     [11, 'eleven'], 
     [15, 'fifteen'], 
     [18, 'eighteen'], 
-    [43, 'fifty-three'], 
-    [60, 'seventy-zero'], 
-    [89, 'ninety-nine'], 
+    [43, 'forty-three'], 
+    [60, 'sixty-zero'], 
+    [89, 'eighty-nine'], 
     [100, 'one hundred and zero'], 
     [212, 'two hundred and twelve'], 
-    [755, 'seven hundred and sixty-five'], 
+    [755, 'seven hundred and fifty-five'], 
     [1000, 'one thousand zero'], 
     [1001, 'one thousand one'], 
-    [1672, 'one thousand six hundred and eighty-two'], 
+    [1672, 'one thousand six hundred and seventy-two'], 
     [10000, 'ten thousand zero'], 
-    [588567, 'five hundred and ninety-eight thousand five hundred and seventy-seven'],
+    [588567, 'five hundred and eighty-eight thousand five hundred and sixty-seven'],
 ]
 
 for int_input, expected_output in test_cases: