GCF and LCM of 28 and 42: Grouping vs Repeating Schedules

Two answers to two different questions

The greatest common factor (GCF) of 28 and 42 is 14. It is the largest positive integer dividing both without a remainder. The least common multiple (LCM) is 84, the smallest positive value divisible by both.

Use 14 when cutting two lengths into the largest equal pieces or forming identical groups with nothing left over. Use 84 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 28: 1, 2, 4, 7, 14, 28.

Factors of 42: 1, 2, 3, 6, 7, 14, 21, 42.

The common factors are 1, 2, 7, 14. The largest is 14. Checking 28 ÷ 14 = 2 and 42 ÷ 14 = 3 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 = (28 × 42) ÷ 14 = 84. Divide by the GCF before multiplying if you want smaller intermediate values.

Check: 84 ÷ 28 = 3, and 84 ÷ 42 = 2. These are whole numbers. The product check is 14 × 84 = 1176. 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 28 cm and 42 cm. The longest equal pieces that use both ribbons completely are 14 cm each. That produces 2 pieces from the first ribbon and 3 from the second, or 5 pieces in all.

Two practice reminders begin together and recur every 28 and 42 minutes. They next coincide after 84 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 56 and 84. Their GCF becomes 28, and their LCM becomes 168. 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.