OG详解-OG5 数学1 Q4

Choice B is correct. The second equation in the given system is r = 3. Substituting 3 for r in the first equation in the given system yields s+7(3) = 27, or s+21 = 27. Subtracting 21 from both sides of this equation yields s = 6. Therefore, the solution (r, s) to the given system of equations is (3, 6).
Choice A is incorrect. This is the solution (s, r), not (r, s), to the given system of equations. Choice C is incorrect and may result from conceptual or calculation errors. Choice D is incorrect and may result from conceptual or calculation errors.