What is GCD(81, 20)?

The GCD (Greatest Common Divisor) is the largest number that can divide two (or more) numbers without leaving a remainder.

The GCD of 81 and 20 is 1.

How to compute GCD(81, 20)

Comparing the divisors of 81 and 20

This first method consists in listing the divisors of the two numbers and then identifying the largest one they have in common.

Divisors of 81:

1, 3, 9, 27, 81

Divisors of 20:

1, 2, 4, 5, 10, 20

We can see from these two lists that the greatest divisor they have in common is: 1

For small numbers, this can be done quickly. However, as numbers increase, the list of potential divisors grows longer, making this method cumbersome and less practical.

Euclid's algorithm

Fortunately, there's a much more efficient method: Euclid's algorithm. It's particularly well-suited to larger numbers. Here's how it works:

  1. Divide 81 by 20. The quotient is 4 and the remainder is 1.
  2. The previous divisor (20) is now the dividend. The remainder (1) is the new divisor. Divide 20 by 1. The quotient is 20 and the remainder is 0.
  3. When you reach a remainder of 0, the last divisor (in this case, 1) is the GCD.