Numbers up to 100

I was a bit surprised when I opened this code and started to read more
of the problem set because it involved writing out the numbers in
a different way than I had put in my plan. Once I got going, I
discovered both ways were pretty similar and this way involves
breaking down the number less and in different ways, such as using the
divide_with_remainder function rather than breaking down the number by
each place value. This method also involves less string combination,
which will make the code shorter. Ultimatley I found the coding to
be pretty straightforawrd and so far am encountering less problems than
compared to some of the labs. This is likely because the increased
time spent on the labs has helped me to master the skills and I am
able to more easily implement them here.

numberwords.py

@@ -15,19 +15,32 @@ TENS_NAMES = [
 ]
 
 def int_under_1000000_to_str(number):
-    return "umm..."
+    return None
         
 def int_under_1000_to_str(number):
-    return "umm..."
+    return None
 
 def int_under_100_to_str(number):
-    return "umm..."
+    if number <20:
+        return int_under_20_to_str
+    elif number <100:
+        prefix = divide_with_remainder(number,10)
+        if number % 10==0:
+           return TENS_NAMES[prefix[0]]
+        else:
+            return (TENS_NAMES[prefix[0]]) + "-" +(DIGIT_NAMES[prefix[1]])
+
 
 def int_under_20_to_str(number):
-    return "umm..."
+    if number <10:
+        return int_under_10_to_str(number)
+    elif number <20:
+        return TWEEN_AND_TEEN_NAMES[number-10]
+    else:
+        return "Error"
 
 def int_under_10_to_str(number):
-    return "umm..."
+    return DIGIT_NAMES[number]
 
 def divide_with_remainder(dividend, divisor):
     """Divides one number by another, using whole-number division. 

nw.py

@@ -4,11 +4,11 @@
 # Ex: python nw.py 145
 
 from argparse import ArgumentParser
-from numberwords import int_under_1000000_to_str
+from numberwords import int_under_100_to_str
 
 parser = ArgumentParser("Print out a number as it is spoken in English.")
 parser.add_argument("number", type=int)
 args = parser.parse_args()
-text = int_under_1000000_to_str(args.number)
+text = int_under_100_to_str(args.number)
 print(text)