If there are ten positive real numbers n1 < n2 < n3 … < n10, how many triplets of these numbers (n1, n2, n3), (n2, n3, n4) … can be generated such that in each triplet the first number is always less than the second number, and the second number is always less than the third number?

a) 45

b) 90

c) 120

d) 180

e) 150

One solving method is the following:

Three numbers can be selected and arranged out of ten numbers in 10P3 ways=10!/7!=10*9*8. Now this arrangement is restricted to a given condition that first number is always less than the second number, and the second number is always less than the third number. Hence three numbers can be arranged among themselves in 3! ways.

Required number of arrangements=(10*9*8)/(3*2)=120

(the source: Winners’ Guide to GMAT Math – Part II)

Pls, can someone explain me, why 10P3 is divided by 3! ? Inasmuch as I understand the denominator denotes that arragements cases of repeated numbers are to be excluded. Take we this into account then 3! in the denominator has to do with cases such as (n1, n1, n1) or (n2, n2, n2) … But, the question asked in this problem is of a different kind. Then what does 3! mean or has 3! actully to do with triplets with repeated numbers which I could not comprehend?

Many thanks beforhand for detailed explanations !!!