Total no of words that can be formed from the letters of the word "THINK" = 5*4*3*2*1=120

--------------------------------------------------------------------------------------------------------------------------------

Let us now try to calculate the no of words (that can be formed from the letters of the word "THINK") in which T and H always appear together.

Consider TH as one entity. Then there are 4*3*2*1=24 ways.

However, TH itself can be permuted in 2*1=2 ways.

Thus there are 24*2=48 words in which T and H always appear together.

--------------------------------------------------------------------------------------------------------------------------------

So, the number of ways in which the word "THINK" can be arranged so that the "T" and the "H" are separated by at least one letter is 120-48=72.