If interested in details about Triangular Numbers, you can refer to following link (though it is not needed and one look at the sequence, you will understand what it is)

https://en.wikipedia.org/wiki/Triangular_number

It is, basically, the following sequence -

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, 300, 325, 351, 378, 406

The challenge is to write a formula which can be put down in any cell and which when dragged down will generate the above sequence.

Hence, if I put the formula in say A1, it should generate the above sequence. Same formula if put in C4 and dragged down, should generate the above sequence. Same formula if put in F8 and dragged down, should generate the above sequence. Hence, the formula should not require any tweaking to generate the sequence in different columns. Same formula should work in any cell.

The solution to the above challenge will be published after a month i.e. on 13-Mar-16.