cs50 readability

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <math.h>

int count_letters(string text);
int count_words(string text);
int count_sentences(string text);

int main(void)
{
    string text = get_string("Text: ");
    //printf("%i \n %i \n %i \n", count_letters(text), count_words(text), count_sentences(text));

    float num_letters = (float) count_letters(text);
    float num_words = (float) count_words(text);
    float num_sentences = (float) count_sentences(text);
    //index = 0.0588 * L - 0.296 * S - 15.8
    //L is the avg of letters per 100 words
    float L = (num_letters / num_words) * 100.0;
    //S is the avg of sentences per 100 words.
    float S = (num_sentences / num_words) * 100.0;

    int index = (int) round(0.0588 * L - 0.296 * S - 15.8);
    //printf("%f %f %f\n", num_letters, num_words, num_sentences);
    //printf("%f %f %i\n", L, S, (int) round(I));
    if (index > 1 && index < 16)
    {
        printf("Grade %i\n", index);
    }
    else if (index < 1)
    {
        printf("Before Grade 1\n");
    }
    else
    {
        printf("Grade 16+\n");
    }
}

int count_letters(string text)
{
    int count = 0;
    for (int i = 0, length = strlen(text); i < length; i++)
    {
        int number = (int) text[i];
        if ((number >= 97 && number <= 122) || (number >= 65 && number <= 90))
        {
            count ++;
        }
    }

    return count;
}
int count_words(string text)
{
    int count = 0;
    for (int i = 0, length = strlen(text); i < length; i++)
    {
        //if at the last character end loop
        if (i == length - 1)
        {
            //current char
            int number = (int) text[i];
            int prev_number = (int) text[i - 1];

            if ((number > 65 && number < 90) || (number > 97 && number < 122))
            {
                count ++;
            }
            else if (prev_number != 32)
            {
                count ++;
            }

        }
        else
        {
            //current char
            int number = (int) text[i];
            //next char
            int next_number = (int) text[i + 1];

            //if the next char is a space
            if (next_number == 32)
            {
                //check if current char is not a space
                if (number != 32)
                {
                    count ++;
                }

            }
        }

    }
    return count;
}

int count_sentences(string text)
{
    int count = 0;
    for (int i = 0, length = strlen(text); i < length; i++)
    {
        int number = (int) text[i];
        if (number == 33 || number == 63 || number == 46)
        {
            count++;
        }
    }
    return count;
}