Multiples don't add.
Addends add, like 13 + 14 + 15 = 42 but I suspect you want factors, specifically prime factors.
2 x 3 x 7 = 42
42 and 84
Just 42.
42 and 84
60 and 42 have no common multiples under 100.
42 and so on
They are 41, 42 and 43.
13, 14, 15
If half of the sum is 90 then the full sum must be 180. The three consecutive multiples of 10 which add up to 180 are 50, 60 and 70.
The only common multiples of 6 & 7 up to 100 are 42 & 84.
21, 42, 63, 84.
21 and 42 are.
Because they're in multiples of three - a quick way to tell if a number is a multiple of three is to add up the digits and see if the digits add up to a multiple of three e.g 576, 5+7+6=18, 1+8=9, 9 is a multiple of three