Combinatorics

9000139302

Level: 
A
The phone number contains nine digits. A witness does not remember the full number, but he remembers that the phone number starts by \(728\), ends by \(01\) and there is no repeating digit in the number. How many phone numbers meet these conditions?
\(120\)
\(320\)
\(520\)
\(720\)

9000139303

Level: 
A
A DJ's playlist contains \(18\) songs. In this list there are \(7\) rap songs, \(5\) oldies and \(6\) rock songs. The opening part should consist of one rap song, two oldies and one rock song. The order of the songs does not matter. Find the number of possible ways how to put the opening together.
\(420\)
\(120\)
\(320\)
\(520\)

9000139305

Level: 
A
There are five rooms with three beds and one room with five beds in a hotel. A group of \(20\) people booked rooms at this hotel. Determine the number of possible choices for the people to the five-bed room.
\(\frac{20!} {5!\; 15!}=15\:504\)
\(20\cdot 3\cdot 5=300\)
\(\frac{20!} {3!\; 5!}=3\:379\:030\:566\:912\:000\)
\(20^{5}=3\:200\:000\)