What is the largest multiple of 3 and 7?

Answer 1

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.

Answer 2

The largest multiple of 3 and 7 can be found by finding the least common multiple (LCM) of the two numbers. The LCM of 3 and 7 is 21. Therefore, the largest multiple of 3 and 7 is 21.