Well its difficult but for mod 9 you could use the divisibilty rule for 9, which is, if the sum of the digits of a number is divisible by 9 then so is the number itself. In this case 801,279,386,104 gives 8+0+1+2+7+9+3+8+6+1+0+4=49 so this number isnt divisible by 9 but 801,279,386,100 is divisible by 9 (as the digits sum up to 45 which is divisible by 9). If you split the number 801,279,386,104 into 801,279,386,100 + 4 u can clearly see that 801,279,386,104 leaves remainder 4 when divided by 9. Hope that helps