A rectangular floor is to be covered by \(2 \times 2\) and \(1\times4\) tiles (everything is arranged). Unfortunately one tile got smashed, but we have one more tile of the other kind available. Can we retile the floor perfectly?
There are 13 green, 15 blue, and 17 red chameleons on an island. Whenever two chameleons of different colours meet, they both swap to the third colour (i.e., a green and blue would both become red). Is it possible for all chameleons to become one colour?
Numbers 1 and 2 are written on a whiteboard. Every day Louise’s friend Zara changes these numbers to their arithmetic mean \(a_m\) and harmonic mean \(h_m\).
(The arithmetic mean of two numbers \(a\) and \(b\) is \(a_m=\frac{a+b}{2}\), and harmonic mean of two numbers \(a\) and \(b\) is \(h_m = \frac{2}{\tfrac{1}{a} + \tfrac{1}{b}}\) ).
(a) At some point Zara wrote \(\frac{941664}{665857}\) as one of the two numbers (it is not known which). What was the other number written on the whiteboard at that time?
(b) Can \(\frac{35}{24}\) be ever written by Zara on the whiteboard?
Using mathematical induction prove that \[1 + 2 + 3 + \dots + n = \frac{n(n+1)}{2}.\]
There are \(n\) lines on a plane, and all the lines intersect at exactly one point. Prove that the lines divide the plane into \(2n\) parts.
There are \(n\) lines on a plane, no two lines are parallel, and no three lines cross at one point. Show that those lines dived the plane into \(\frac{n(n+1)}{2}+1\) regions.
In a sequence 2, 6, 12, 20, 30, ... find the number
(a) in the 6th place
(b) in the 2016th place.
Using mathematical induction prove that \[1 +3 +5 +\dots + (2n-1) = n^2.\]
Circles and lines are drawn on the plane. They divide the plane into non-intersecting regions, see the picture below.
Show that it is possible to colour the regions with two colours in such a way that no two regions sharing some length of border are the same colour.
Numbers \(1,2,\dots,n\) are written on a whiteboard. In one go Louise is allowed to wipe out any two numbers \(a\) and \(b\), and write their sum \(a+b\) instead. Louise enjoys erasing the numbers, and continues the procedure until only one number is left on the whiteboard.
What number is it? What if instead of \(a+b\) she writes \(a+b-1\)?