Consider the number 2751. The sum of its digital is `2+7+5+1=15`. Adding the digits of 15 we get `1+5=6`. This number 6 is called the 'digital root' of the number 2751. That is, to find the digital root of a number, find the sum of its digits (Don't forget to find the sum of the digits again, if the first sum has more than one digit)
Let us see one more example.
The sum of the digits of the numbe `679412` is `6+7+9+4+1+2=29`. Sum of digits of `29=2+9=11`. Sum of digits of `11=1+1=2`. Therefore the digital root of `679412` is `2`.
Digital roots have an interesting property. To see this, consider the product `43times27=1161`. The digital roots of the numbers `43` and `27` are `4+3=7` and `2+7=9`. Product of the digital roots=`7times9=63`. Digital root of `63=6+3=9`. The Digital root of 1161 is also `9(1+1+6+1=9)` that is the digital root of `1161`=The digital root of `63`, where 63 is the product of the digital roots of 43 and 27. This property is true for all other natural numbers.
If the digital root of `a` is 5 and the digital root of `b` is 2. Then what is the digital root of `ab`?