Without dividing or listing multiples how can you tell if 61 is a multiple of 6

It's not even. And every multiple if 6 is even.

The answer is 6 weeks

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)