Combinatorics

9000139309

Level: 
A
There are \(20\) tablets in an e-shop. From this amount \(18\) tablets are new and \(2\) tablets have been returned by customers. The e-shop manager gets an order containing three tablets and he wants to get rid of the returned tablets first. How many possibilities exist to complete the order?
\(18\)
\(\frac{18!} {3!\, 15!}=816\)
\(18\cdot 16\cdot 3=864\)
\(20\cdot 19\cdot 18=6\:840\)

9000139701

Level: 
A
There are \(15\) athletes in an athletic meeting. Determine in how many ways it is possible to obtain the results on the first six places of the scoreboard if the place on scoreboard cannot be shared (one athlete per one place on scoreboard).
\(\frac{15!} {9!} =3\:603\:600\)
\(6^{15}=470\:184\:984\:576\)
\(15!\, 6!=941\:525\:544\:960\:000\)
\(\frac{15!} {9!\, 6!}=5\:005\)

9000139310

Level: 
A
There are \(20\) tablets in an e-shop. From this amount \(18\) tablets are new and \(2\) tablets have been returned by customers. The e-shop manager gets an order containing three tablets and he wants to use only the new tablets for this order. How many possibilities exist to complete the order?
\(\frac{18!} {3!\, 15!}\)
\(18\)
\(18\cdot 16\cdot 3\)
\(20\cdot 19\cdot 18\)

9000139707

Level: 
A
A Morse code utilized dots and dashes to encode letters of an alphabet. Find the number of signals of the length from \(1\) to \(4\) which can be obtained from dots and dashes.
\(2 + 2^{2} + 2^{3} + 2^{4}=30\)
\(1 + 2 + 3! + 4!=33\)
\(\frac{4!} {3!\, 2!}=2\)
\(2 \cdot 1 + 2 \cdot 2 + 2 \cdot 3 + 2 \cdot 4=20\)

9000139708

Level: 
A
The shelf contains \(15\) books. From this amount, \(9\) books are in English and \(6\) books in other languages. Find the number of possibilities how to rearrange the books on the shelf, if all English books have to be on the left and the other on the right.
\(9!\, 6!=261\:273\:600\)
\(9^{6}=531\:441\)
\(\frac{9!} {6!}=504\)
\(\frac{9!} {6!\, 3!}=84\)

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.
\(420\)
\(120\)
\(320\)
\(520\)

9000139710

Level: 
C
The wallet contains nine coins: three \(1\)-Euro coins, three \(2\)-Euro coins and three \(5\)-Euro coins. How many different amounts can be paid if we have to pay the amount exactly and use just three coins for this payment?
\(\frac{5!} {3!\, 2!}=10\)
\(\frac{5!} {3!}=20\)
\(3^{3}=27\)
\(3!=6\)