16 Dec
2011
16 Dec
'11
3:18 p.m.
4. You're on your own about dividing the 3-digit number by 7 and 13, but you could just memorize all multiples of 7 and 13 up to 999 for instant result or just subtract out some obvious multiple like 130, 260, and then you only need to memorize multiples of 13 up to 130.
it's easier than that. to find out if a number is divisible by 7, take the last digit, double it, and subtract it from the rest of the number. the new one is divisible by 7 when the old one is. to find out if a number is divisible by 13, take the last digit, multiply it by 4, and add it to the rest of the number. the new one is divisible by 13 when the old one is. erich