Solutions
Suppose the sequence of 3n consecutive integers begin with integer m.
Then the 3n consecutive integers are:
m, m + 1, m + 2,…………, m + (3n - 1)
Out of these integers, 3 integers can be chosen in 3nC3 ways.
Let us divide these 3n consecutive integers in the groups G1, G2 and G3 as follows:
G1: m, m + 3. m + 6, …….., m + (3n - 3)
G2: m + 1, m + 4, m + 7, ……., m + (3n - 2)
G3: m + 2, m + 5, m + 8, …….., m + (3n - 1)
The sum of 3 integers chosen from the given 3n integers will be divisible by 3 if either all the three integers are chosen from the same group or one integer is chosen from each group. The number of ways that the three integers are from the same
