Find a number which is multiplied by 2 when you shift the units digit to the other end of the number.
The numbers 1, 1, 2, 2, 3, 3, ... n, n are to be arranged in a sequence in such a way that for each r = 1, 2, ... n, the two r's are separated by exactly r places. An example, with n = 4, is:
4 1 3 1 2 4 3 2
Such a sequence is called a Langford Sequence (see The Mathematical Gazette (1958) p.228).
Find a Langford Sequence with n = 8.
Problem of the Year
Can you continue this list, expressing 7, 8, 9 etc in a similar way? The rules of the game are that you must use all the digits 1,9,9 and 7 in that order, and the symbols +, -, ×, ÷, square roots and brackets.
Any of the Above?
Exactly one of the following five statements is true. Which one?
All of the following
None of the following
Some of the following
All of the above
None of the above
Find four perfect squares, two consisting of three digits, one of two digits, and one of one digit, using each of the numbers 1,2,3,...,9 just once.
There are nine coins in South African currency:
1c, 2c, 5c, 10c, 20c, 50c, R1, R2, R5
Combinations of these coins cover all transactions, either by direct payment or by payment involving change.
For example, an item costing R1.98 can be paid with seven coins:
R1 + 50c + 20c + 20c + 5c + 2c + 1c
or by a transaction involving only two coins: a payment of R2 and 2c change.
Of all amounts under R10, which involves the largest number of coins, either as direct payment or one involving change?
Problems: Prime reciprocals
Any rational number can be expressed as a repeating decimal. The length of the repeating segment is called the period of the decimal. So 1/7 = 0.142857142857... has period 6, while 1/11 = 0.9090909... has period 2.
What prime numbers have reciprocals with period five or less? This problem can be tackled crudely with a computer, but a technology-free solution is also possible.
The following problem appeared in the Swedish magazine Elementa 4(95):
Visa att radien till den inskrivna cirkeln hos en pythagoreisk triangel (= en rätvinklig triangel med hetalssidor) är hetalsvärd.
Can you solve it?
From Foggott to Urr
The roads are all straight and level on the plains of Oblivia. Some distances in kilometers are given in the guide:
How far is it from Fogott to Urr?
Z W E I
+ D R E I
F Ü N F
Can you replace the letters by the digits 1,2,3,...,9 to make the addition correct? Each letter stands for a different digit.
Seven coins are placed in a circle, with heads up. A move consists of flipping five coins in a row. Using only this move, can you make all the coins show tails simultaneously? What is the smallest number of moves needed?