Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add hamzabellouch/agent-skills --skill arithmetic-and-number-sensegit clone --depth 1 https://github.com/hamzabellouch/agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/hamzabellouch/agent-skills/arithmetic-and-number-sense)<a href="https://agentmods.dev/skills/hamzabellouch/agent-skills/arithmetic-and-number-sense"><img src="https://agentmods.dev/badge/skills/hamzabellouch/agent-skills/arithmetic-and-number-sense/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/hamzabellouch/agent-skills/arithmetic-and-number-sense"><img src="https://agentmods.dev/badge/skills/hamzabellouch/agent-skills/arithmetic-and-number-sense.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00057 | $0.00538 |
| Opus 5.5 | $0.00023 | $0.00215 |
| Sonnet 5.5 | $0.00011 | $0.00108 |
| Haiku 4.5 | $0.00006 | $0.00054 |
Grade A, and why
arithmetic-and-number-sense scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured today.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
What it actually says
Arithmetic & Number Sense
Core skills
- Order of operations (PEMDAS/BODMAS): parentheses, exponents, multiplication/division left to right, addition/subtraction left to right.
- Fractions: a/b + c/d = (ad + bc)/bd; multiply straight across; divide by multiplying by the reciprocal; simplify with gcd.
- Percent: x% of y = xy/100; percent change = (new - old)/old x 100; successive changes multiply (+20% then -20% = -4%).
- Ratios/proportions: a/b = c/d -> ad = bc; unit rates; scale factors; inverse proportion xy = k.
- Exponents: a^m a^n = a^(m+n); (a^m)^n = a^(mn); a^0 = 1; a^(-n) = 1/a^n; a^(1/n) = nth root.
- Scientific notation: a x 10^n with 1 <= a < 10; multiply mantissas and add exponents.
- Divisibility: 2 (last digit even), 3 (digit sum), 4 (last two digits), 5, 9 (digit sum), 11 (alternating sum).
- Rounding and significant figures; estimation by rounding to 1-2 digits; bound the error.
Mental-math tricks
- x5: halve then x10. x11: add neighboring digits. Square ending in 5: n5^2 = n(n+1) followed by 25.
- (a+b)(a-b) = a^2 - b^2 for near-equal products (47 x 53 = 50^2 - 3^2 = 2491).
- Percent swap: x% of y = y% of x.
Word-problem method
- Underline the unknown. 2. Assign a variable with units. 3. Translate sentences to equations. 4. Solve. 5. Check in the original wording and sanity-check the magnitude.
Pitfalls
Dividing by a fraction, negative signs with exponents (-3^2 = -9 vs (-3)^2 = 9), percent of vs percent more than, unit conversions.
What ships with it
1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- today First seen · 28 lines · 57 tokens per session scan A fd9ce3689e86
arithmetic-and-number-sense is a skill published in the GitHub repository hamzabellouch/agent-skills (8 stars, last pushed today), licensed MIT. It adds 57 tokens to every session and 538 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-10-17.
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