Formula that doesn’t work??

So I’m trying to do my ged stuff and I’ve run into a major question. When calculating density, the formula is D=M/V. That works for the most part, but if it’s V your missing, it doesn’t work intuitively requiring you to do multiple steps to have D and V switch sides for it to work rather than just dividing M on both like every other problem. I have found the triangle thing that does work, I just can’t wrap my mind around why the formula doesn’t always work the way most problems do