If 50 apprentices can finish a job in 4 hours, and 30 journeymen can finish the same job in 5 hours, how much of the job should be completed by 10 apprentices and 15 journeymen in one hour? Choose the correct choice.
A) 1 / 9	B) 29 / 180	C) 26 / 143	D) 3 / 20	E) 39 / 121

Rationale: "50 apprentices can finish the job in 4 hours , thus : 10 apprentices can finish the job in 4 * 5 = 20 hours ; in 1 hour 10 apprentices can finish 1 / 20 of the job . 30 journeymen can finish the same job in 4,5 hours , thus : 15 journeymen can finish the job in 5 * 2 = 10 hours ; in 1 hour 15 journeymen can finish 1 / 10 of the job . therefore , in 1 hour 10 apprentices and 15 journeymen can finish 1 / 20 + 1 / 10 = 3 / 20 of the job . answer : d ."
Answer: D