Math (HELP!)

Okay. I'm feeling kinda right now cuz i should know this but i don't. I actually know how to get the anwser...but i need and expression. (Algebra)

Okay, Their is a set of stairs made up of blocks. For three steps, its 6 blocks. The question is asking how many i would need for a 20 step staircase, and i could draw it out...but i don't want to do it that way. I'm trying to get and expression. When i put in into an Output Inpout chart, i see that the output numbers increse by one each time...but i don't know what to do after that!
Could somebody please help me a.s.a.p!

Thanks.......i'm still working on it. Sry for any spelling or grammar that is really bad---i was in a rush!

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  1. step(1) = 1
    step(2) = 3
    step (3) = 6
    ..
    ..
    step(n) = n(n+1)/2

    so step(20) = 20(21)/2 = 210

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