Question: pipes a and b can fill a tank in 6 and 4 hours . pipe c can empty it in 12 hours . if all pipes are opened together , then the tank will be filled in ?
A) 3 hr
B) 2 hr
C) 1 hr
D) 9 / 7 hr
E) 5 / 3 hr
Correct answer: A
Grade: 7 | Topic: Ratios and proportions | Difficulty: Hard | Standard: 7.RP.A.3