How many 4-digit positive integers are there, where each digit is positive, and no 3 adjacent digits are same?
Choose one:
A) 1236 B) 3528 C) 4096 D) 4608 E) 6561
First digit.. 9 posibilities   Second digit, 8 possibilities   third digit, 7 possibilities   Fourth digit, 7 possibilities.   9*8*7*7= 
<gadget id="calculator">9*8*7*7</gadget>
<output>3_528</output>
3528.   3528
<result>B</result>
How many 4-digit positive integers are there, where each digit is positive, and no 3 adjacent digits are same?