I was having difficulties with this one as well->

A combination lock shows 30 different numbers on the dial. To open the lock, the user must know the three-number sequence that is the "combination" for the lock. For example, to open the lock one might have to dial the three numbers 12-2-14 in the order shown here (first dialing 12, then 2, then 14). In a combination, you can't have two consecutive numbers that are the same. For example, you can have 7-10-7 but you can't have 7-7-10.

How many different combinations are possible?