Explanation:
The digital root of a number is the single digit that you get by adding all of the digits of the original number together. If the result of that is multiple digits, you add those digits together, repeating the process until you get a single digit. That digit is the digital root of the original number.
example given
257520643
2+5+7+5+2+0+6+4+3
7+12+2+10+3
7+1+2+2+13
8+4+1+3
8+4+4
12+4
16
1+6=7