How many 3-digit numbers can be formed using the digits 1, 2, 3, 4, 5, 6, 7 if repetition of digits is not allowed?

Think of it this way:
_ _ _ (3 digit number)

for the first blank you could have 7 possibilities
for the second blank you could have 6 possibilities
for the third blank you could have 5 possibilities
because each time you fill in a blank one number is removed from the list of possibilities

7×6×5=210. there are 210 possibilities