Readability

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

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

int main (void)
{
    string user_input = get_string("Text:");
    int letters = count_letters(user_input);
    int words = count_words(user_input);
    int sentences = count_sentences(user_input);

    double grade = 0.0588 * (((float)letters / words) * 100) - 0.296 * (((float)sentences / words) * 100) - 15.8;

    if(grade >= 16)
    {
        printf("Grade 16+");
    }
    else if(grade < 1)
    {
        printf("Before Grade 1");
    }else
    {
        printf("Grade: %i\n", (int)round(grade));
    }

}

int count_letters(string text)
{
    int letters = 0;

    for (int i = 0, length = strlen(text); i < length; i++)
    {
        if(isalpha(text[i]) != 0)
        {
            letters++;
        }
    }
    return letters;
}

int count_words(string text)
{
    int words = 0;

    for (int i = 0, length = strlen(text); i < length; i++)
    {
        if(isspace(text[i]) != 0)
        {
            words++;
        }
    }
    return words += 1;
}

int count_sentences(string text)
{
    int sentences = 0;

    for (int i = 0, length = strlen(text); i < length; i++)
    {
        if(text[i] == ',' || text[i] == '.' || text[i] == '!'|| text[i] == '?')
        {
            sentences++;
        }
    }
    return sentences;
}