I am so confused at the moment as I can't seem to figure out whether MRTS (i.e. the slope of the firm's isoquant) is MPL/MPK or MPK/MPL. At first I thought I cleared it up as it would probably depend on the axes of the graph (I think), but then I find that both are used just as much no matter where I look. For instance, I have shown two images below. One from my lecture and another from a google search.
Similarly, is the slope of the isocost therefore w/r or r/w (where r is price of capital and w is price of labour)?