OG详解-OG6 数学1 Q17

Choice D is correct. It’s given that the values of P, N, and C are positive. Therefore, dividing each side of the given equation byPN​​ = 19-C. Subtracting 19 from each side of this equation side of this equation yields PN​​ -19= -C. Dividing each side of this equation by -1 yields 
19−PN​=C​，or C= 19−PN​.
Choice A is incorrect. This equation is equivalent to P = NC-19, not P = N(19-C). Choice B is incorrect.  This equation is equivalent to P = 19-NC, not P = N(19-C). Choice C is incorrect. This equation is equivalent to P = N(C-19), not P = N(19-C).