There are 5 odd digits: 1, 3, 5, 7, 9

How many numbers with all odd digits are between 100 and 1000?

Isn't this just how many 3 digit numbers have all odd digits?

# 3 odd digit numbers = (# possible odd digits for 1st digit) x (# possible odd digits for 2nd digit) x (# possible odd digits for 3rd digit)

= 5 x 5 x 5 = ...