# GCD and modulo arithmetic

> gcd(48,18)? What is (7^10) mod 13 via repeated squaring?

Canonical: https://learn.significanthobbies.com/curriculum/drills/gcd-modular

gcd(48,18)? What is (7^10) mod 13 via repeated squaring?

## The mechanism

Interviews use math sparingly: gcd/lcm, modular arithmetic, primes, combinatorics, and basic geometry (cross product, line intersection). Recognize the shape of the problem — the formula is a quick lookup away.

## Expected evidence

gcd=6; 7^10 mod 13 = 4.

## Hints

- Euclidean gcd.
- Binary exponentiation reduces mod each step.

## Sources and practice

- [Computational geometry (Wikipedia)](https://en.wikipedia.org/wiki/Computational_geometry)
- [Study Math & Geometry](https://learn.significanthobbies.com/curriculum/concepts/math-geometry)
- [Try the interactive drill](https://learn.significanthobbies.com/drills/gcd-modular)
- [All practice briefs](https://learn.significanthobbies.com/curriculum/drills/)