A satellite is composed of 24 modular units, each of which is equipped with a set of sensors, some of which have been upgraded. Each unit contains the same number of non-upgraded sensors. If the number of non-upgraded sensors on one unit is 1/4 the total number of upgraded sensors on the entire satellite, what fraction of the sensors on the satellite have been upgraded?
Pick.
A) 5 / 6	B) 1 / 5	C) 1 / 6	D) 1 / 7	E) 1 / 24

Rationale: "let x be the number of upgraded sensors on the satellite . the number of non - upgraded sensors per unit is x / 4 . the number of non - upgraded sensors on the whole satellite is 24 ( x / 4 ) = 6 x . the fraction of sensors which have been upgraded is x / ( x + 6 x ) = x / 7 x = 1 / 7 the answer is d ."
Answer: D