readability test

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

int count_letters(string t);
int count_words(string t);
int count_sentences(string t);
int formula(int l, int w, int s);

int main(void)
{
    string text = get_string("Text: ");

    int l_ = count_letters(text);
    int w_ = count_words(text);
    int s_ = count_sentences(text);
    int index = formula(l_, w_, s_);

    if (index >= 16)
    {
        printf("Grade 16+\n");
    }
    else if (index < 16 && index >= 1)
    {
        printf("Grade %i\n", index);
    }
    else
    {
        printf("Before Grade 1\n");
    }
}

int count_letters(string t)
{
    int letters = 0;
    int len = strlen(t);
    for(int x = 0; x < len; x++)
    {
        if (isalpha(t[x]) != 0)
        {
            letters++;
        }
    }
    return letters;
}

int count_words(string t)
{
    int spaces = 0;
    int len = strlen(t);
    for(int x = 0; x < len; x++)
    {
        if (t[x] == 32)
        {
            spaces++;
        }
    }
    return spaces + 1;
}

int count_sentences(string t)
{
    int sentences = 0;
    int len = strlen(t);
    for(int x = 0; x < len; x++)
    {
        if(t[x] == 33 || t[x] == 46 || t[x] == 63)
        {
            sentences++;
        }
    }
    return sentences;
}

int formula(int l, int w, int s)
{
    float L = (l / w) * 100;
    float S = (s / w) * 100;
    float index = 0.0588 * L - 0.296 * S - 15.8;
    return round(index);
}