Question

It takes 6 cubes to build a staircase with 3 steps.how many cubes will be needed for 11steps

Answer 1

1 step - 1 cube
2 steps - 3 cubes
3 steps - 6 cubes
etc.

The number of cubes needed for a staircase with $n$ steps is equal to $1+2+3+\ldots+n$.

So, if $n=11$
the number of cubes is $1+2+3+4+5+6+7+8+9+10+11=55$