10 men,working 6 hours a day can complete a work in 18 days.How many hours a day must 15 men work to complete the same work in 12 days ?	Pick:
A) 6  B) 7  C) 8  D) 9  E) 10

Rationale: "solution let the required number of hours per day be x . more men , less hours per day ( indirect proportion ) less days , more hours per day ( indirect proportion ) ∴ 15 × 12 × x = 10 × 18 × 6 ⇔ x = 10 x 18 x 6 / 15 x 12 = x = 6 . answer a"
Answer: A