Quiz Question

Number 212 of a series

The circles represent four pennies arranged in a square. All right, they don’t look like pennies, but use your imagination. Also use your imagination to move two pennies to form another square smaller than the original.

Post your solution in the comments section below.

Update and Solution

Everybody who had a go at this one got it. Here’s the picture.

Quiz Question

Number 209 of a series

What’s the last digit of the following?

171999 + 111999 – 71999

Enter your answer in the comments section below.

Update and Solution

Mike saw through this one early, only his comment was so cryptic I missed it. See below. I didn’t see it until I started working through it, then I thought, “too easy.” Here is how it works.

A is the last digit of 171999. B is the last digit of  111999. C is the last digit of 71999.

It is obvious A = C, so the answer is B. And B is 1.

Quiz Question

Number 206 of a series

This one is from The Moscow Puzzles by Boris A. Kordemsky, page 44.

That’s supposed to be a chess board above, although I have not shaded in the black and white squares. It doesn’t matter, because this is not a real chess game. The black circles are black pawns, about twice as many as in a real chess game. The question is can you set a white knight on the board and proceed to capture all 16 pawns in 16 consecutive moves? Let’s assume you can, then what are the moves?

Post your answer in the comments section below.

Update and Solution

Mike comments this is too easy. Could be. Kordemsky has this to say:

Start by capturing one of the pawns not marked with a red dot in the diagram below.

Then the rest should be straight-forward.

Quiz Question

Number 205 of a series

I’m late posting this week’s Quiz Question. Monday was a busy day. But here it is. I came across an item posted on the Internet—The Subtle Art of the Mathematical Conjecture. It has some interesting stuff, including proved and yet to be proved conjectures in mathematics. There is a discussion of Fermat’s Conjecture, and there is this:

Counterexamples can lie far ashore, like the one found by Noam Elkies, a mathematician at Harvard University, disproving Euler’s conjecture, a variation on Fermat’s conjecture that states that a fourth power can never be written as a sum of three other fourth powers. Who would have guessed that the first counterexample involved a number of 30 digits?*

The asterisk points to a note:

20,615,6734 = 2,682,4404 + 15,365,6394 + 18,796,7604.

Am I the only one who sees something wrong with this equation? Post your comments below.

Update and solution

Mike and Elmo got this one right. I had it wrong. I tried to check the math in my head and miscounted the multiplications. Shit happens.

Quiz Question

Number 204 of a series

Math time again. Some time ago I had the idea to clear out shelf space, so I dumped all the copies of Schaum’s Outline books that are available on Kindle. This is from Schaum’s Vector Analysis, Chapter 2.

2.11. Show that the vectors A = − i + j, B = − ij – 2k, C = 2j + 2k form a right triangle.

i, j, and k are the unit vectors parallel to the 3D coordinate axes. To be clear, imagine each vector as a straight line oriented in space but movable. Can you move the three lines into position to form a right triangle? Give a mathematical proof.

The answer is in the book, but you know better than to look for the answer. Post your answer as a comment below.

Update and solution

First it is needed to demonstrate the three vectors satisfy the Pythagorean Theorem. Square the lengths of all vectors and see if the sums of two add up to the third. So we have:

|A|² = 2

|B|² = 6

|C|² =8

That works.

To form a closed polygon (triangle) the sum of the vectors must be zero.

Sum = (-1 -1)i (1 -1 +2)j (-2 +2)k, which is not zero. But if you reverse A, then

Sum = (1 -1)i (-1 -1 +2)j (-2 +2)k, which is zero.

Also, note the dot product of A and B is zero. The two vectors are orthogonal.

So the three vectors form a right triangle.

Quiz Question

Number 200 of a series

Here is a nice problem, not too difficult, pertinent to a current hot topic.

Hypothetical scenario: Nothing is adding carbon dioxide to the atmosphere. Carbon dioxide has a 100-year half life in the atmosphere. We crank up a contraption that pumps 100 million tons of carbon dioxide into the atmosphere each year. How much carbon dioxide is in the atmosphere when  a steady state is obtained?

Post your answer as a comment below. Extra points for describing the calculation.



Quiz Question

Number 199 of a series

This one is from The Moscow Puzzles by Boris A. Kordemsky, page 45.

Yes, this is the time-honored eight queens problem. There is a star on square A2 of the chess board. Place seven more stars on the board so no two stars are on the same row, column, diagonal.

Add your solution as a comment below. Just identify the squares where the next seven stars need to go.

Quiz Question

Number 198 of a series

This one is from The Moscow Puzzles by Boris A. Kordemsky, page 43.

There are 25 numbered checkers in 25 squares. You are allowed to exchange checkers in pairs. How many exchanges are required to put all checkers in numerical order, left to right, top row to bottom row?

Post your answer as a comment below.

Update and solution:

I was unable to solve this one, so I went to the answers in the back of the book. From page 216 (not verbatim):

The minimum solution is 19 moves.

1 -7, 7 – 20, 20 – 16, 16 – 11, 11 – 2, 2 – 24

3 – 10, 10 – 23, 23 – 14, 14 – 18, 18 – 5

4 – 19, 19 – 9, 9 – 22

6 – 12, 12 – 15, 15 – 13, 13 – 25

17 – 21

Quiz Question

Number 196 of a series

The box is a square with sides = 1. The circular arcs are tangent to the sides of the square. What is the area of the colored area?

Post your answer in the comments section below.

Update and solution:

Here is another rendition of the above. What is the area of the blue regions?

If one of the inscribed circles were complete, the square would cut out ¼ of the circle. The area of the the circle covered by the square is ¼ of the circle: π/4. The area of one blue region is 1 – π/4. The combined blue area is 2 – π/2. The area of the orange section is 1 – 2 + π/2 or π/2 – 1.

Quiz Question

Number 195 of a series

The image shows two circles with inscribed rectangles. The top rectangle is a square. The bottom rectangle is two squares. Are the two rectangle equal area? Describe a proof and post your answer in the comments section below.

Update and solution:

Notice what happens when you cut the big square in half and move half of it to a new position, as I have done imperfectly here.

Quiz Question

Number 194 of a series

Above you see the well-known Tower of Hanoi puzzle. I have depicted it here as eight disks, of ascending size, stacked on an upright pole set into a game board. The object is to transfer all the disks from where they are to one of the other poles. You can move one disk at a time, and you must place it on one of the poles, on the bare board or else on a larger disk already there.

  1. How do you do it? What sequence of moves accomplishes the objective?
  2. How many moves are required?
  3. This shows eight disks. How long would it take, moving one disk a second, if there were 64 disks?

Post your answer in the comments section below.