I灯泡 = P/U = 4W/6V = 2/3 A
因为电路串联,所以电流处处相等.
R总 = U总/I = 15V/（2/3)A = 22.5欧姆
R灯泡 = U^2/P= (6V)^2/4W = 9欧姆
所以 R =R总 - R灯泡 = 22.5欧姆 - 9欧姆 = 13.5欧姆
答：……