If a three digit number can be divided by 3, just see if the sum of all digits of the number can be divided by 3. Why? Is there such a rule if a four digit number can be divided by 3?

If a three digit number can be divided by 3, just see if the sum of all digits of the number can be divided by 3. Why? Is there such a rule if a four digit number can be divided by 3?

So, for example, a three digit 123 can be decomposed into 99 + 1 + 18 + 2 + 3
Because 99 and 18 are multiples of 3, we only consider 1 + 2 + 3
For another example, 514 is decomposed into 5 × 99 + 5 + 9 + 1 + 4. Because both 99 and 9 can be divisible by 3, only 5 + 1 + 4 is considered. If they can be divisible by 3, then the three digits can be divisible by 3
Four figures are also true