Looking up tiles

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Okay im having trouble working out the math of looking up a tile from a bitmap file. The bitmaps are all 32 by 32 and there can be any amount of tiles in them in any number of rows or columns. I plan on the function taking the number of columns and the number for the tile say 5. Then I should be able to find the screen coordinates in pixels of the tile. Tile one would have a top of 0 a bottom of 32 a left of 0 and a right of 32. It seems easy but im having trouble any ideas? or did i not explain it well enough.