GCF and LCM of 72 and 90: Grouping vs Repeating Schedules

Two answers to two different questions

The greatest common factor (GCF) of 72 and 90 is 18. It is the largest positive integer dividing both without a remainder. The least common multiple (LCM) is 360, the smallest positive value divisible by both.

Use 18 when cutting two lengths into the largest equal pieces or forming identical groups with nothing left over. Use 360 when looking for a common repeating interval. Factors divide the starting numbers; multiples are produced by multiplying them.

Find the GCF from complete factor lists

Factors of 72: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72.

Factors of 90: 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90.

The common factors are 1, 2, 3, 6, 9, 18. The largest is 18. Checking 72 ÷ 18 = 4 and 90 ÷ 18 = 5 confirms that neither division leaves a remainder.

Find the LCM and verify it

For positive integers, GCF × LCM = the product of the two numbers. Therefore LCM = (72 × 90) ÷ 18 = 360. Divide by the GCF before multiplying if you want smaller intermediate values.

Check: 360 ÷ 72 = 5, and 360 ÷ 90 = 4. These are whole numbers. The product check is 18 × 360 = 6480. Simply multiplying the inputs gives a common multiple, but here it is not the least one because they share a factor greater than 1.

Apply each result to a different situation

Two ribbons measure 72 cm and 90 cm. The longest equal pieces that use both ribbons completely are 18 cm each. That produces 4 pieces from the first ribbon and 5 from the second, or 9 pieces in all.

Two practice reminders begin together and recur every 72 and 90 minutes. They next coincide after 360 minutes, assuming exact intervals and no interruptions. If they start at different times, you need their starting offsets as well; the LCM alone does not settle that problem.

Try a scaled example

Double both starting numbers to 144 and 180. Their GCF becomes 36, and their LCM becomes 720. The number of ribbon pieces in the grouping example remains the same because all lengths were scaled equally.

Before calculating, name the unit and decide whether you are seeking a divisor or a shared multiple. That choice is usually more important than the arithmetic itself.

Check the mathematical definitions

For fractions, percentages and worked arithmetic, see OpenStax Prealgebra. For number definitions, see OpenStax on prime and composite numbers and Wolfram MathWorld on digital roots. Worked examples on this page are calculated for this guide.