how many 3 - digit positive integers are there , where each digit is positive , and no 3 adjacent digits are same ?
"first digit . . 9 posibilities second digit , 8 possibilities third digit , 7 possibilities 9 * 8 * 7 = 504 . b"
multiply(multiply(add(3, 3), add(3, 3)), multiply(add(3, 3), multiply(3, 3)))