Hi Shy,
Since there are seven different letters and we want to find 5 letter arrangements, there are 6 ways that the first letter can be selected, 5 for the 2nd, 4 for the 3rd, 3 for the 4th without choosing the letter t. Since the last choice must be a letter t, there is only one possibility. Therefore the number of arrangements of five letters that contains the letter t is calculated by multiplying 6 X 5 X 4 X 3 X 1 = 360
