ForHosting KIT · Developer Utilities

Casting out nines check

Casting out nines is a classical arithmetic check that uses congruence modulo nine to catch mistakes in addition and multiplication without redoing every digit by hand.

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Given two or more integer operands, a claimed sum or product, and an operation flag, this capability reduces each number to its residue class modulo nine, combines those residues the same way the original operation would, and compares the result with the residue of the claim. When the two sides disagree, the claimed answer is certainly wrong. When they agree, the claim is only consistent with modular arithmetic: a transposition or an error of a multiple of nine can still hide. The engine is pure and deterministic, runs free in the browser for teaching demos, and costs $0.002 per successful API item when you need the same check from scripts, graders, or homework helpers.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/numth/casting-out-nines

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/numth/casting-out-nines \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"operands":[123,456],"claimed":579,"operation":"sum"}'

const res = await fetch("https://api.kit.forhosting.com/numth/casting-out-nines", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "operands": [
      123,
      456
    ],
    "claimed": 579,
    "operation": "sum"
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/numth/casting-out-nines",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "operands": [
        123,
        456
    ],
    "claimed": 579,
    "operation": "sum"
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/numth/casting-out-nines", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"operands":[123,456],"claimed":579,"operation":"sum"}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"operands":[123,456],"claimed":579,"operation":"sum"}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/numth/casting-out-nines", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "operands": [
    123,
    456
  ],
  "claimed": 579,
  "operation": "sum"
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "numth.casting_out_nines",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →