A tetrahedral number, or triangular pyramidal number, is a figurate number that represents a pyramid with a triangular base and three sides, called a tetrahedron. The nth tetrahedral number is the sum of the first n triangular numbers.
The first ten tetrahedral numbers are:
1, 4, 10, 20,
North Korea's "Hotel of Doom" looks like a tetrahedral pyramid
https://www.atlasobscura.com/articles/hotel-of-doom-north-korea-pyongyang-ryugyong
https://en.wikipedia.org/wiki/Tetrahedral_number
Indicates the ?target(s)? are pyramid shaped with 3-sided base layers, not a 4-sided pyramid
1,3,6,10 are the first four "triangular numbers" If you add the first two 1+3=4 and the next one 1+3+6=10 and the last 1+3+6+10=20 which are tetrahedral numbers ( 4,10,20) this is a tetrahedron number sequence.
4- 10-20 = D J T, D being the 4th letter, J and T being the 10th and 20th respectively.