Media Prep Math Practice

A self-contained web app that drills microbiology students on the math behind preparing dehydrated culture media. Students see a randomized problem, get three attempts to enter the correct mass in grams, and on either success or failure are shown a full step-by-step ratio-and-proportion solution.

Built for Preliminary Microbiology lab instruction. Drop it on GitHub Pages and share the link — no server, no build step, no dependencies.

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Live demo

Once deployed, the app lives at: https://calebhendren.github.io/mediacalc/

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Features

• Randomized problems on every page load. The pool size scales with your media list: by default 1,905 unique combinations (381 volumes × 5 media types). The footer recomputes the count automatically when you edit the media list.
• 75/25 volume mix. 75% of the time the prompt uses a "clean" classroom volume (100, 150, 200, 250, 300, 400, 500, 600, 750 mL, 1 L, 1.5 L, 2 L); 25% of the time it uses a truly random value snapped to the nearest 5 mL.
• Three attempts per problem, with a visual pip indicator showing remaining attempts.
• Worked solution. Reveals after a correct answer or three wrong attempts, walking through the proportion x g / V mL = gPerL g / 1 L, the unit conversion, the cross-multiplication, and the final 2-decimal-place answer.
• Mental-math shortcuts for the four common volumes where one is useful: 100 mL (move the decimal), 250 mL (÷4), 500 mL (÷2), and 2 L (×2).
• Liter display for ≥ 1 L volumes. Questions show "1.5 L" instead of "1500 mL"; the solution starts with an explicit L-to-mL conversion step.
• On-screen calculator. Always visible on desktop as a sticky sidebar; collapsible on mobile via a "Show calculator" button. Supports keyboard input (digits, operators, Enter, Backspace, Escape) when the answer field is not focused.
• New question button for unlimited practice without reloading the page.
• Accessibility.
  • Semantic HTML landmarks and ARIA live regions for feedback.
  • Skip link, visible focus rings, ≥ 44 × 44 px touch targets.
  • Every equation has a role="math" and a plain-English aria-label so screen readers describe the math instead of trying to parse the visual fraction layout.
  • prefers-reduced-motion respected.
• Light and dark themes. Follows the browser/OS color scheme: dark by default, and the Chattanooga State light theme (navy, royal blue, orange — shared with MicroGrade) when light is preferred.

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How a problem is graded

The student's input is parsed as a number and compared to the true answer with a tolerance of ±0.02 g (configurable). This is loose enough to accept either round-half-up or round-half-down at the 2-decimal place, but tight enough that random guesses fail.

For example, for 1635 mL of MSA the exact answer is 181.485 g. Any of the following is accepted:

• 181.49
• 181.48
• 181.485

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Project structure

.
├── index.html           # Markup only — no inline CSS or JS
├── css/
│   └── styles.css       # All styles
├── js/
│   ├── config.js        # Media list and tunable constants — edit this
│   ├── utils.js         # Formatting and random helpers
│   ├── question.js      # Random question generation
│   ├── solution.js      # Worked-solution HTML builder
│   ├── calculator.js    # On-screen calculator widget
│   └── app.js           # Entry point — wires everything together
└── README.md

The JS is loaded as native ES modules (<script type="module">), which every browser shipped in the last several years supports. There is still no package.json, no bundler, no build step.

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Customizing the media list

Everything you might want to change lives in a single file: js/config.js.

Add, remove, or edit media types

export const MEDIA = [
  { name: 'nutrient agar',      gPerL: 23  },
  { name: 'tryptic soy agar',   gPerL: 40  },
  { name: 'citrate agar',       gPerL: 24  },
  { name: 'MacConkey agar',     gPerL: 50  },
  { name: 'mannitol salt agar', gPerL: 111 }
];

• name is rendered directly in the problem text. Use the spelling and capitalization you want students to see (the app capitalizes the first letter at the start of a sentence).
• gPerL is grams of dehydrated powder per 1 L of finished medium.
• You can have any number of entries (1 or more). The footer's "X unique problems across N media types" line updates automatically.

These default values reflect common Difco/BD formulations; verify against your own manufacturer's label before using in the actual lab.

Other tunable constants (same file)

Constant	Default	What it controls
COMMON_VOLUMES_ML	[100, 150, 200, 250, 300, 400, 500, 600, 750, 1000, 1500, 2000]	The "clean" classroom volumes used 75% of the time.
COMMON_VOLUME_PROBABILITY	0.75	Probability (0–1) of picking from COMMON_VOLUMES_ML instead of the random range.
RANDOM_MIN_ML / RANDOM_MAX_ML / RANDOM_STEP_ML	100 / 2000 / 5	Random-volume range and step size in mL.
ANSWER_TOLERANCE_G	0.02	Maximum allowed error in grams when grading a student's answer.
ATTEMPTS_PER_QUESTION	3	Number of guesses before the solution is revealed. The pip indicator scales to match.

No other file needs to change. The footer count, attempts pip count, and grading tolerance all derive from these values at runtime.

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Local development

Because the app uses ES modules, opening index.html directly via file:// will be blocked by the browser's module CORS rules. Run it through a local static server instead:

# Python 3
python3 -m http.server 8000

# Node (with npx, no install needed)
npx serve .

Then visit http://localhost:8000.

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Deployment to GitHub Pages

1. Push this repository to GitHub.
2. Go to Settings → Pages.
3. Under Source, choose the main branch and the / (root) folder.
4. Save. After a minute or two, the site will be live at https://calebhendren.github.io/<repo-name>/.

GitHub Pages serves the files over HTTPS, so the ES modules load with no extra configuration. No build step or GitHub Actions required.

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Browser support

Tested on current versions of Chrome, Safari, Firefox, and Edge, on both desktop and mobile. The app uses no build tooling and no transpilation; it relies on ES2017+ features and native ES modules, which every browser released since 2018 supports.

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Accessibility notes

This app is intended for use in higher-education courses and aims to meet WCAG 2.1 Level AA.

• All interactive controls are keyboard-reachable with visible focus indicators.
• Color is never the sole indicator of state; feedback messages also include text and an icon.
• Touch targets meet the 44 × 44 px minimum.
• Form fields have explicit <label> associations, validation messages are surfaced via an ARIA live region, and equation blocks expose plain-English descriptions for screen readers.
• A skip link allows keyboard users to jump past the header to the practice problem.

If you find an accessibility issue, please open an issue on the repository.

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License

MIT. Use it, modify it, share it with your students.