Guess the odds of drawing blue

guessBlue.php

<?php
/**
 * Unit tests for Blue and Red Marbles game.
 *
 * @see     https://www.codewars.com/kata/thinkful-number-drills-blue-and-red-marbles/
 *
 * You and a friend have decided to play a game to drill your statistical intuitions.
 * The game works like this:
 * You have a bunch of red and blue marbles.
 * To start the game you grab a handful of marbles of each color and put them
 * into the bag, keeping track of how many of each color go in.
 * You take turns reaching into the bag, guessing a color, and then pulling one
 * marble out. You get a point if you guessed correctly.
 * The trick is you only have three seconds to make your guess,
 * so you have to think quickly.
 *
 * You've decided to write a function, guess_blue() to help automatically calculate
 * whether you should guess "blue" or "red". The function should take four arguments:
 *  the number of blue marbles you put in the bag to start
 *  the number of red marbles you put in the bag to start
 *  the number of blue marbles pulled out so far, and
 *  the number of red marbles pulled out so far.
 *
 * guess_blue() should return the probability of drawing a blue marble,
 * expressed as a float.
 *
 * @package Cata\Tests\Unit\NumberDrills
 * @author  Caspar Green <[email protected]>
 * @since   1.0.0
 */
declare(strict_types=1);

namespace Cata\Tests\Unit\NumberDrills;

use Cata\NumberDrills\GuessMarblesGame;
use PHPUnit\Framework\TestCase;

/**
 * Class GuessMarblesGameTest
 *
 * @package Cata\Tests\Unit\NumberDrills
 * @since   1.0.0
 */
class GuessMarblesGameTest extends TestCase
{
    /**
     * Test guessBlue() should return odds of drawing a blue marble.
     *
     * @since 1.0.0
     *
     * @return void
     */
    public function testGuessBlueShouldReturnOddsWhenBlueAndRedMarbles(): void
    {
        $game = new GuessMarblesGame();

        $this->assertEquals(0.0, $game->guessBlue(0, 0, 0, 0));
        $this->assertEquals(0.6, $game->guessBlue(5, 5, 2, 3));
    }
}

<?php
/**
 * GuessMarbleGame class.
 *
 * @package Cata\NumberDrills
 * @author  Caspar Green <[email protected]>
 * @since   1.0.0
 */
declare(strict_types=1);

namespace Cata\NumberDrills;

/**
 * Class GuessMarblesGame
 *
 * @package Cata\NumberDrills
 * @since   1.0.0
 */
class GuessMarblesGame
{
    public function guessBlue(
        int $initialBlue,
        int $initialRed,
        int $drawnBlue,
        int $drawnRed
    ): float {
        $remainingBlue  = $initialBlue - $drawnBlue;
        $remainingTotal = $remainingBlue + $initialRed - $drawnRed;

        return $remainingTotal
            ? $remainingBlue / $remainingTotal
            : 0;
    }
}