I'm unsure how to approach this equation using the ideal gas law; P=KT/V. P=80, T=100, V=5.

80 = 100K/5 ---- Did I set up the equation right?

As far as the math goes this looks fine to me, so you can solve for K given values for P, T, and V. The ideal gas law is can be written in the form PV=nRT, so here your K is equal to the number of moles of gas (n) times the ideal gas constant (R).