21
7 its self is the smallest multiple of 7 and is also 2 x 3.5.
738
21 has 3 as a factor and is a multiple of 7, and so as 210, 105, and an infinite number of other numbers.
I'm not sure what would be a trick for the number 3. I will presume you would mean the way to tell if a number is a multiple of 3. A number is a multiple of 3 if the sum of its digits is also a multiple of 3. For example 585 is a multiple of 3 since' 5+8+5 = 18 which is a multiple of 3. The number 47 is not a multiple of 3 because 4+7 equals 11 and that is not a multiple of 3. There is a second digit that has a similar rule for multiplicity. That digit is 9, which just happens to equal 32 . In fact, since 18 is a multiple of 9, 585 must be a multiple of 9 also.
The factors of 21 are 1, 3, 7, 21. The factors which are not a multiple of 7 are 1 and 3.
Any number that is not a multiple of 7.Any number that is not a multiple of 7.Any number that is not a multiple of 7.Any number that is not a multiple of 7.
42. 2*3*7=42.
The Least Common Multiple (LCM) for 3 5 7 8 is 840.
42
There is no such number. Since if x were the largest multiple of 3 and 7 then what about 2x? 2x would be a multiple of 3 since x is a multiple of 3; 2x would be a multiple of 7 since x is a multiple of 7; and 2x is bigger than x. So x cannot be the largest.
The (LCM) Least Common Multiple of 7 and 3 is 21 because that is the LEAST number that both 3 and 7 go into.
Since 19 is a prime number, it is not divisible by 3 or 7.