Showing what's behind the curtain

Find how many 18-digit numbers (without leading zeros) there are such that no digit occurs more than three times in base 10.

Find the number of triangles present in a cross-hatched triangle of size n.

Maximizing a weighted product

Project Euler 183: Find the maximum product of equal parts of a number and determine whether the that product is a terminating or non-terminating decimal .

Find the number of consecutive integers pairs that have the same number of positive divisors.

Find the last digits of a tetration calculation.

Counting the number of “hollow” square laminae that can form one, two, three, … distinct arrangements.

Using up to one million tiles find many different “hollow” square laminae can be formed.

Find hexadecimal numbers containing at most 16 hexadecimal digits exist with all of the digits 0, 1 and A present at least once

RSA encryption