Collected Problems 2

I'm collecting various problems especially the short lived ones from twitter here for future use. My taste runs towards geometry as is probably evident. For ease of use I've hidden my notes. Click the button below to expose them.

Problem 1 [**]

There are 100 people in line to board a plane with 100 seats. The first person has lost their boarding pass, so they take a random seat. Everyone that follows takes their assigned seat if it's available, but otherwise takes a random unoccupied seat. What is the probability the last passenger ends up in their assigned seat? 


Provable via induction start with the case of  3 passengers.

Problem 2 [***]

Find the ratio of the area of the red triangle to the green quadrilateral.

Find the ratio of the radius of the incircle of a triangle to the its height in terms of its side lengths: a,b, and c.  Use Heron's formula. r/h=b/2s

Problem 3 [*]

[Simon Pampena]

How many squares of Area A can fit inside square of area B.

Problem 4 [*]

If you flip a particular unfair coin 6 times, P(3H & 3T) = 16%. If you flip the same coin 4x, what is P(2H & 2T)?

[Matt Enlow]

Don't forget about the number of combinations i..e 6!/(3!*3!)  = 20

Problem 5 [***]

[Henk Reuling]

This is quite tricky and requires calculus.
1. Derive the triangle ratio 2:1
2. Take derivative of ellipse ax^2 + y^2 = 100 and set to -1/2 at tangent to find pt (x,2ax)
3. Use similar triangles to find width is 4ax + x - 10 which gives a  2nd point
4. Plug both into the ellipse equation and solve for a to get 4/9
5. Plug the point (x,2ax) in to get (9,8) and calc the length.

Problem 6 [*]

ABCD is a 6x8 rectangle where the length of AB is 8, and length of BC is 6, and the 2 
subsegments in the cross are 2 and 3.

What's the area of EFGH?

Problem 7 [**]


In the  figure, triangle ABC is a right angle isosceles triangle with angle A = 90 degrees, and square ACDE is a parallelogram. When AE = 5 cm, BE = 8 cm, how many cm^2 is the area of ​​pentagon ABCDE?

Square off the figure by extending AE. That produces congruent triangles on both sides.
You can then find a formula for the total area 1/2(a^2+b^2) + 5a and a
Pythagorean equivalent (a+5)^2 + b^2 = 8^2.

Problem 8 [**]


1. The geometric approach is to extend chord and realize the max area is at its perpendicular
bisector. You can then easily angle chase and see this goes through the center and is also a 1:2.
2. If you have the linear alg. / calculus computing the area via vectors and differentiating is fairly
interesting as a secondary exercise.

Problem 9 [*]

"I have 7 sticks all of different lengths and all an integer length long. The longest one is shorter than a piece of paper and therefore under 30 centimeters.  Whenever I pick 3 sticks, I notice I can't form a triangle. What is the length of the shortest stick?"


Problem 10 [**]

Given the following areas, find the area X of the remaining piece of the triangle.


  • Use proportional thinking.
  • Draw a line from the top to the intersection and setup 2 proportional equations.

Problem 11 [*]



  • Extend ED to intersect with BC and an isosceles triangle will emerge.

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