The first few common multiples of 4 and 6 are 12, 24, 36, 48, … The smallest one (12) has its own name in later grades: the least common multiple.
Why multiples are useful
Multiples turn up everywhere:
Schedule problems. A bus comes every 5 minutes, another every 7 minutes. They leave together at 8:00. When will they both be at the stop at the same time? — find a common multiple of 5 and 7. The answer is 35 minutes later, at 8:35.
Dividing into groups. You can split 36 sweets evenly among 4, 6, 9 or 12 children, because 36 is a multiple of all those numbers.
Spotting multiples at a glance
Some multiples are easy to spot from the digits:
Multiples of 2 end in 0, 2, 4, 6 or 8.
Multiples of 5 end in 0 or 5.
Multiples of 10 end in 0.
Multiples of 9 have digits that add up to a multiple of 9 (you'll learn this trick more in Year 5).