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Consider the integer powers of 7: a(n) = 7^n:
7, 49, 343, 2401, 16807,... 117649, ...

Prove that the second from right digit in all those numbers (excluding the 1st) is either 0 or 4.

(No Solution Yet, 1 Comments) Submitted on 2018-12-16 by Ady TZIDON    

Three concentric circles each have radii 8, 15 and 17.

Find the largest possible area of the equilateral triangle with one vertex on each circle.
(No Solution Yet, 2 Comments) Submitted on 2018-12-15 by Danish Ahmed Khan    

Begin with a finite sequence of blocks in a row, each in one of 3 colors: red, blue, yellow.

Below each pair of neighboring blocks place a new block with the color rule: If the blocks are the same color use that color but if they are different use the third color.

Example:

r b y y b
 y r y r
  b b b
   b b
    b
How can the color of the last block be easily predicted from the top row?

Note: I don't know the full answer but can solve special cases.

(No Solution Yet, 4 Comments) Submitted on 2018-12-14 by Jer    

Difficulty: 3 of 5 Discs in Triangle (in Geometry) Rating: 3.00
Find the side length of the smallest equilateral triangle in which three discs of radii of 2, 3, 4 can be placed without overlap.
(No Solution Yet, 2 Comments) Submitted on 2018-12-13 by Danish Ahmed Khan    

What is the highest prime number with no repeated digits?

(Solution Posted, 2 Comments) Submitted on 2018-12-12 by Ady TZIDON    

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