Answer:
28, 35, 42 are consecutive multiples of 7.
95, 100, 105 are consecutive multiples of 5
a(n), a(n+1), a(n+2) are consecutive multiples of a.
Take a list of numbers that all have the same factor in common, divide it out. The result should be consecutive numbers.
28, 35, 42 can be divided by 7, the results are 4, 5, and 6. These are consecutive numbers.
95, 100, 105 can be divided by 5, the results are 19, 20 and 21. These are consecutive numbers.