How can I subdivide this plane into even squares? I have a plane that has a scale of x: 100, y: 5, z: 1, and I want to split it into squares that have a scale of x: 5, z: 5, but when I do subdivide, I get a result like this: 



When in reality, I'd rather have something like this:



How can I do this? (Sorry for the badly sketched pictures.)
Use loop cuts, press Ctrl+R and use the scroll wheel to select how many. Hover over the edge you want the cuts to intersect, then double-click to finish the action.